Step out of your car at the Seltún geothermal field (63.8968, -22.0552) along Route 42 in the heart of Krýsuvík, and if you happen to be wearing a polished silver ring on your finger, take a close look at the metal before you walk onto the wooden boardwalk. By the time you return forty-five minutes later, after climbing past the hissing fumaroles, the blood-red clay banks, and the slate-blue cauldrons of boiling mud, that bright silver band will have turned dull grey-black, while the heavy, metallic scent of hydrogen sulfide (H2S) will have worked its way so deep into the wool of your coat that you will still smell it in your hallway two days later.

You did not just look at a geothermal hillside. You walked through an open-air chemical reactor where the Mid-Atlantic Ridge is digesting its own basalt crust in real time.

In the spoken episode, we trace the diagnostic arc from the 2,000-year-old coastal clinker of Katlahraun, past the screaming summer bird wall of Krýsuvíkurberg and the emerald blast crater of Grænavatn (63.8854, -22.0626), up to the boiling acid slopes of Seltún and the silent black water of Kleifarvatn (63.9286, -22.0403). What the audio runtime leaves behind is the human and scientific story buried inside that terrain: how German chemist Robert Bunsen used Seltún in 1846 to figure out how volcanoes manufacture sulfuric acid, why a clogged borehole from the 1940s blew a visitor coffee shed off the map in 1999, how an 1151 AD lava flow spared a Viking-Age church inside the green island of Húshólmi, and what divers found on the vibrating floor of Kleifarvatn after an earthquake pulled the plug on the lake in June 2000.

Quick Facts: Krýsuvík, Seltún & Lake Kleifarvatn

Attribute	Verified Field Data
Canonical Names & Etymology	Krýsuvík (/ˈcʰriːsʏˌviːk/, "Krýsa's Bay") \| Seltún ("Shieling / Summer-Pasture Meadow") \| Kleifarvatn ("Lake of the Cliffs/Clefts") \| Grænavatn ("Green Lake") \| Húshólmi ("House Islet") (Visit Reykjanes).
Key Coordinates & Dimensions	Seltún: 63.8968, -22.0552 (>200°C at <1 km depth) \| Kleifarvatn: 63.9286, -22.0403 (~9.1–10 km², 97 m deep) \| Grænavatn Maar: 63.8854, -22.0626 (45 m deep) \| Krýsuvíkurberg: 63.8340, -22.1000 (15 km long, 40–70 m high) (Visit Reykjanes; Iceland Loop).
Geochemical Color Code	Yellow: Native sulfur (S) sublimating from H2S \| White/Grey: Kaolinite/smectite clay, amorphous silica (SiO2), anatase & gypsum left by sulfuric acid (H2SO4) leaching \| Red/Orange: Oxidized hematite (Fe2O3) & goethite \| Slate Blue-Grey: Anaerobic pyrite (FeS2)-bearing clay slurry (Markússon & Stefánsson 2011; Visit Reykjanes: History of Seltún).
Explosions & Tectonic Draining	Autumn 1999: Clogged 230 m borehole Drottningarhola (KV-14) exploded at Seltún, blasting a ~30–43 m crater (with repeat blowouts in 2010 & 2019). June 17 & June 21, 2000: Two Mw 6.5 South Iceland earthquakes (with dynamically triggered fault slip at Kleifarvatn on June 17, 2000) cracked Kleifarvatn's bed, draining ~20% of its surface area (~12% volume) (Visit Reykjanes; Pagli et al. 2003).
Human & Archaeological Layers	Húshólmi: Viking-Age farm and church ruins spared as a kipuka inside the 1151 AD Ögmundarhraun lava (Ferlir). Summer 1846: Chemist Robert Bunsen sampled Seltún's acid fumaroles and mud pools (often misdated 1845 in secondary guides) (Bunsen the Geochemist). Krýsuvíkurkirkja: Built 1857, burned by arson in 2010, rebuilt by Hafnarfjörður students in 2020 (Iceland Loop).
Ecosystem & Extremophiles	Biomarkers in an acidic 87°C Krýsuvík mud pot indicate contributions from archaea and other microorganisms associated with geothermal environments (Sánchez-García et al. 2020); Krýsuvíkurberg hosts ~57,000–60,000 breeding pairs of seabirds each summer (Visit Reykjanes).
Literature & Screen Credits	Arnaldur Indriðason's novel Kleifarvatn (The Draining Lake, 2004); Darren Aronofsky's Noah (2014, Kleifarvatn shore); Black Mirror ("Crocodile", 2017, Kleifarvatn road); David Guetta ft. Sia's "She Wolf" (2012, Seltún).

Robert Bunsen's Glass Ampoules and the Borehole That Blew Up the Coffee Shed

After Mount Hekla erupted in September 1845, the Danish Crown invited the German chemist Robert Bunsen, the scientist whose laboratory burner still sits on every high-school chemistry bench, to join a research expedition to Iceland (Bunsen the Geochemist). Sailing north in May 1846 for a four-month field campaign (an expedition often misdated to 1845 in travel guides and Wikipedia), Bunsen rode across the Reykjanes lava fields to Krýsuvík in early summer 1846 carrying eudiometers and blowtorch-sealed glass ampoules to collect gases and fluids directly from the smoking vents of Seltún (63.8968, -22.0552).

What Bunsen set out to explain in his landmark 1847–1851 geochemical papers was a puzzle that had long baffled European geologists: how does steam rising through dark, solid volcanic rock turn an entire mountainside at the foot of the subglacial ridge of Sveifluháls into soft, multicolored clay? Less than a kilometer beneath the boardwalk, magmatic dykes heat groundwater above two hundred degrees Celsius. As that pressurized fluid flashes to steam and rises along fractures, it carries dissolved hydrogen sulfide (H2S) gas to the surface. Where H2S meets atmospheric oxygen right at a hot vent lip, it precipitates into fragile, lemon-yellow needles of pure elemental sulfur (S). Where the gas dissolves into shallow pools of condensing steam and rainwater, where microbial communities can also participate in sulfur transformations, it oxidizes into hot sulfuric acid (H2SO4).

Field mineralogical studies at Krýsuvík (Markússon & Stefánsson 2011) show how that sulfuric acid strips calcium, magnesium, sodium, and iron out of the dark basalt and yellowish-brown palagonite tuff (móberg), bleaching the rock into a white-and-grey skeleton of kaolinite and smectite clays, amorphous silica (SiO2), anatase (TiO2), and gypsum. Out on the open, oxygen-exposed banks, the leached iron rusts into deep blood-red hematite (Fe2O3) and ochre-orange goethite. Down inside the thick, oxygen-starved boiling mud cauldrons, however, reducing conditions cause iron to bond with sulfur instead of oxygen, saturating the pale clay slurry with finely divided black pyrite (FeS2, iron sulfide) that gives the bubbling mud pots their signature slate-blue and blue-grey color (Markússon & Stefánsson 2011; Visit Reykjanes: History of Seltún; Wikipedia: Seltún Hot Springs).

For two hundred years, people tried to cash in on Seltún's chemical reactor, and every time the hillside ate their tools. In the eighteenth and nineteenth centuries, Danish Crown administrators and British concessionaires sent workers into Seltún and nearby Brennisteinsfjöll to scrape native sulfur out of the scalding mud and haul it by packhorse to Hafnarfjörður for gunpowder manufacture (Visit Reykjanes). Sulfuric acid corroded iron spades in weeks, and heavy H2S gas pooling in windless hollows threatened miners whose noses had gone numb from olfactory fatigue after twenty minutes in the fumes.

Twentieth-century drilling rigs fared no better. In the late 1940s, engineers drilled exploratory steam wells at Seltún hoping to heat Hafnarfjörður, only to abandon the field around 1950 when acid steam corroded the pipes (Visit Reykjanes). Half a century later, in autumn 1999, a 230-meter-deep relic from that drilling campaign known as Drottningarhola ("Queen's Borehole", logged as KV-14) clogged shut with precipitated minerals. Ten days after the well plugged, subterranean steam pressure blew the cap in a violent phreatic explosion, excavating a boiling crater roughly thirty to forty-three meters across, obliterating a nearby visitor refreshment shed, and raining scalding clay across the parking lot, with smaller borehole blowouts repeating the warning in 2010 and 2019 (Visit Reykjanes; Wikipedia: Krýsuvík). When you walk Seltún's boardwalk today, never step off the planks onto the white-and-yellow silica crust: it is a brittle crème-brûlée shell over 80–100°C acid mud that will severely burn a foot in an instant, and in winter freezing steam coats the wood in black glaze ice requiring traction cleats.

The 45-Meter Steam Bomb and the Lake That Pulled Its Own Plug: Grænavatn & Kleifarvatn

One kilometer south of Seltún along Route 42, the rust-red soil opens into a startling turquoise-green crater called Grænavatn (63.8854, -22.0626), flanked by two smaller water-filled hollows known as Augun ("The Eyes") (Visit Reykjanes). In sunny snapshots, Grænavatn looks like a gentle swimming lagoon; in reality, it is a forty-five-meter-deep maar: an explosion crater blasted into the bedrock more than 6,000 years ago when rising basaltic magma intersected shallow groundwater and flashed it into steam expanding sixteen hundred times in volume, hurling deep crustal gabbro xenoliths (hnyðlingar) onto the rim while suspended sulfur compounds, silicates, and thermal algae dyed the deep water emerald green.

Drive two kilometers north of Seltún in the opposite direction and the sulfur plumes give way to the silent black beaches of Kleifarvatn (63.9286, -22.0403). Covering roughly nine to ten square kilometers and plunging ninety-seven meters deep between the serrated palagonite walls of Sveifluháls and Vatnshlíð, Kleifarvatn has no surface rivers flowing into it and no rivers flowing out (Visit Reykjanes). Its surface is simply the exposed window of the peninsula's groundwater table sitting inside a fractured basalt sieve.

In June 2000, two major magnitude 6.5 earthquakes struck the South Iceland Seismic Zone four days apart: the first on June 17, 2000 and the second on June 21, 2000 (Wikipedia: 2000 Iceland earthquakes). As radar interferometry and field surveys later demonstrated (Pagli et al. 2003; Clifton et al. 2003), seismic waves from the June 17 mainshock dynamically triggered a sharp fault-slip earthquake beneath Kleifarvatn within five minutes, tearing open mineral-sealed fractures across the lake floor. Over the following months, Kleifarvatn quietly drained into the crust, losing roughly twenty percent of its surface area (about twelve percent of its total water volume, dropping its level by more than two to four meters) before fine sediment gradually resealed the cracks over subsequent years (Visit Reykjanes; Wikipedia: Kleifarvatn).

As the dark water retreated, it exposed wide black sand flats later used by Darren Aronofsky to film the pre-Flood wasteland of Noah (2014) and by Netflix's Black Mirror for the winter road scenes of "Crocodile" (2017), and inspired Icelandic novelist Arnaldur Indriðason to write his Cold War mystery Kleifarvatn (The Draining Lake, 2004), in which the receding shoreline uncovers a human skeleton anchored to the bottom by a Soviet listening device. When scientific divers and multibeam sonar teams mapped the southern floor of Kleifarvatn in 2012 and 2013, they also found the physical engine behind the old folkloric tales of the Kleifarvatnsskrímsli: the whale-sized black serpent monster long said to disturb the surface (Visit Reykjanes): four submerged volcanic ridges and six underwater hot-spring craters streaming curtains of CO2 and H2S gas bubbles so forcefully that divers could feel the bedrock vibrating through their gloves.

The Unburned Island in the Lava and the Chapel Built Twice: From Húshólmi to Krýsuvíkurkirkja

Because almost every traveler turns around after driving past Kleifarvatn and walking the Seltún boardwalk, very few follow the volcanic corridor south toward the ocean, where the human history of Krýsuvík has spent nine hundred years refusing to be erased by lava, surf, or fire.

Down on the Atlantic shore west of the 1880 dry-stone fishing huts of Selatangar (haunted in fishermen's lore by the restless ghost Tanga-Tómas) sits Katlahraun (63.8400, -22.2100), a two-thousand-year-old coastal lava pond that dammed against the surf and then drained away, leaving towering basalt walls and hollow gas chimneys (hornitos) (Visit Reykjanes). Immediately east rises the fifteen-kilometer-long, forty-to-seventy-meter-high sea cliff of Krýsuvíkurberg (63.8340, -22.1000), where ten stacked layers of ancient lava shelter 57,000 to 60,000 breeding pairs of kittiwakes, guillemots, razorbills, fulmars, and puffins every summer before the ledges empty into winter silence in late August (if you walk the cliff top, stay well back from the grassy lip, where hidden puffin burrows undermine the turf above a fifty-meter drop).

Between those cliffs and the inland hills once stood the wealthy medieval manor estate (höfuðból) of old Krýsuvík, located near the coast above the cove now called Hælsvík (Visit Reykjanes). In local folklore, the estate took its name from a fierce settler-woman named Krýsa, whose bitter boundary dispute with her neighbor Herdís of Herdísarvík allegedly cursed the local trout to grow hair (loðsilungur) and called down a storm on Krýsa's fishing boats before both women dropped dead on the mountain track where two stone cairns still mark their graves.

In 1151 AD, during the twelfth-century Krýsuvík Fires (Krýsuvíkureldar), the earth delivered a real verdict (Ferlir). A volcanic fissure unzipped along the eastern flank of the Núpshlíðarháls ridge and sent the vast, jagged basalt sheet of Ögmundarhraun pouring down to the sea, burying the coastal pastures of old Krýsuvík under meters of black stone (Visit Reykjanes). Yet as the molten river swept toward the surf, a rise in the ground split the lava stream in two, sparing a green, vegetated island of older earth, a geological kipuka, called Húshólmi ("House Islet", 63.8560, -22.1450) alongside its sister islet Óbrynnishólmi ("Unburned Islet"). Hike across the rough clinker of Ögmundarhraun into the grassy clearing of Húshólmi today and you stand inside a Viking-Age time capsule sealed by twelfth-century fire: stone foundations of an early medieval longhouse (skáli), turf boundary walls, a churchyard, and the ruins of one of the oldest churches in Iceland, stratified with the 871±2 AD Settlement ash layer (Landnámslagið) (Ferlir).

Driven inland when Ögmundarhraun swallowed their coastal meadows in 1151, the families of Krýsuvík rebuilt their manor on the green slope just south of Seltún. There, in 1857, they raised the tiny wooden chapel of Krýsuvíkurkirkja (63.8880, -22.0640) (Iceland Loop). Long after the last resident sheep farmers left the estate in the twentieth century and the land passed to the town of Hafnarfjörður, the little 1857 timber church stood unlocked on the moor as a quiet refuge for travelers, until the night of January 2, 2010, when arsonists set it on fire and burned it to the ground.

In most places, a burned nineteenth-century country chapel in an abandoned valley would have been left as a brass plaque on a stone foundation. Instead, carpentry teachers and students at the technical college in Hafnarfjörður spent the next ten years milling timber by hand to the exact 1857 measurements, and in 2020 they crane-lifted the rebuilt Krýsuvíkurkirkja back onto its stone plinth below the steaming slope of Seltún (Iceland Loop). Walk down from the boiling acid vents to that small wooden church before you leave the valley, lay your palm against the fresh timber wall while steam from the 1999 borehole crater drifts across the grass, and you understand the real bargain of living on the Reykjanes rift: the crust can digest its own basalt into clay, swallow a fifth of a lake, and encircle a medieval farm in molten stone, and people will still come back to rebuild the roof.

A STORY. NOT A GUIDE.
ICELAND. TOLD.

Frequently Asked Questions: Krýsuvík, Seltún & Kleifarvatn

Why does Seltún in Krýsuvík have so many bright colors and smell like rotten eggs?

At Seltún (63.8968, -22.0552), groundwater superheated above 200°C rises to the surface carrying hydrogen sulfide (H2S) gas, which smells like rotten eggs and tarnishes silver. As H2S reacts with oxygen and shallow water, it can produce yellow native sulfur and sulfuric acid (H2SO4): an acid bath that bleaches dark basalt into white kaolinite clay, amorphous silica, and anatase, rusts iron into red hematite (Fe2O3) and orange goethite on oxygen-exposed slopes, and forms slate-blue / blue-grey pyrite (FeS2)-bearing clay slurries in anaerobic boiling mud pots (Markússon & Stefánsson 2011; Sánchez-García et al. 2020; Visit Reykjanes: History of Seltún).

What happened when Lake Kleifarvatn drained after the June 2000 earthquakes?

Kleifarvatn (63.9286, -22.0403, 97 m deep) has no surface rivers; its level reflects the local groundwater table inside porous volcanic rock. During the June 17 and June 21, 2000 South Iceland earthquake sequence (specifically after dynamic fault slip was triggered beneath Kleifarvatn within five minutes of the June 17 Mw 6.5 mainshock), newly opened fissures across the lake floor drained roughly 20% of the lake's surface area (~12% of its volume) into the crust before sediment gradually resealed the cracks over subsequent years (Pagli et al. 2003; Clifton et al. 2003; Wikipedia: Kleifarvatn).

What is Grænavatn, and why is its water bright green?

Located 1 km south of Seltún along Route 42, Grænavatn (63.8854, -22.0626) is a 45-meter-deep phreatomagmatic explosion crater (maar) blasted into the bedrock over 6,000 years ago when rising magma hit groundwater and flashed it into steam. Its vivid turquoise-green color comes from sunlight scattering off suspended sulfur and silicate particles alongside thermal algae in the deep crater water (Visit Reykjanes).