The glacier near Vík: Mýrdalsjökull, and the volcano underneath it

Almost every mix-up in this niche is a mix-up between two ice caps. Iceland has a much bigger one further east, Vatnajökull, and that is the ice behind the photographs people picture when they hear the words: a smooth blue chamber, a rippled ceiling, somebody in a red jacket for scale. Those caves sit two to three hours east of here, at Skaftafell and around the lagoon at Jökulsárlón, and they are a cold-season thing. The glacier near Vík is a different ice cap with a different story.

What Mýrdalsjökull is. It is an ice cap rather than a valley glacier: a broad dome of ice sitting on high ground, with tongues running down off its edges. Snow lands on the top, is pressed into ice by the snow that lands on it next, and creeps outward and downward under its own weight. That slow movement is what opens the channels and chambers a guide can take a group into. The glacier near Vík is the fourth largest of these caps in the country, which makes it big enough to hold a cave and close enough to a village that you can be back for lunch.

And the volcano part. Katla lies under the ice, and the geology of the two is the reason this cave looks the way it does. Layer after layer of dark ash has settled on the snow over the centuries and been buried by the next winter, so the ice is banded: grey, black, translucent, grey again. Guides read those bands the way you read tree rings. It is the single most photographed thing about the glacier near Vík, and the operator of $274 names the setting outright — a drive through the Katla Geopark to the cave in Mýrdalsjökull.

What nobody can promise. No listing on this site promises blue walls, a dry floor or an empty chamber, and neither do we. Meltwater carves the space, and the space changes shape from month to month; the guide picks the route on the morning. What the fare buys you is a qualified guide, the helmet and the crampons, a vehicle that can get across the sand, and access to ground you cannot reach on your own.

Between the village and the ice there is a plain of black sand, and it is the reason every listing here includes a vehicle transfer rather than a meeting point at the cave. The surface is soft, braided with meltwater channels and re-drawn by weather, so it is driven in high-clearance super jeeps on fat tyres. That is transport, not the attraction: it gets you to the edge of the ice on a glacier near Vík that has no car park of its own.

What the half day looks like. You meet in the village, change into the jeep, and spend the middle of the trip climbing the sand and then the ice margin. The operator hands out a helmet and crampons at the edge, gives the short briefing about walking with your feet further apart than feels natural, and walks the group in. $250 describes exactly that shape — through the black sand dunes to the cave with a local guide, then back to Vík — in three hours, rated 4.8. $298 stretches the same day to four hours and adds glacier canyons, hidden waterfalls and time for photographs.

What the full day adds. The listings that start in Reykjavik are the same cave with a coastline attached. $285, our pick, runs twelve hours and puts Skógafoss, Seljalandsfoss and the black sand of Reynisfjara around the cave visit; $274 does the same in ten hours with fewer people, and $291 runs it as a small-group minibus day with the jeep leg included, rated 4.9. If you are already staying on the coast, the half day is the better use of your hours; if you are based in the capital, the long day saves you a night.

What to bring. Boots you have walked in, windproof layers and gloves. The technical kit comes from the operator — the listings say so themselves. Standing on the edge of a glacier near Vík is colder than the weather in the village suggests, because the wind comes down off the ice and keeps coming.