Key takeaways
- Four hundred feet from the crater floor to the rubble at the bottom.
- The entrance is a hole a few metres across in the floor of the crater.
- The walls lean away as you descend, so the floor is far broader than the opening.
- The site's own board scales it against a church tower and the Statue of Liberty.
The dimensions
The descent is about 120 metres, which is four hundred feet, from the crater floor to the rubble pile at the bottom of the chamber. That is the number the tour is built on and it is the one worth fixing in your head before you go, because from the rim there is nothing to give it away.
The opening is small. It is a hole in the floor of the crater a few metres across, fenced and roped, spanned by a steel gantry with a platform hung beneath it. Standing next to it you would not guess what is underneath; it looks like a fissure, of which the lava field has many.
Below the opening the space widens considerably. The shaft belongs to the neck of the volcano and it flares outwards as it goes down, so the floor is a far larger area than the entrance, and the walls curve away from the platform as it descends. That widening is the single most disorienting thing about the ride down: you expect a shaft and you get a room.
What it compares to
A hundred and twenty metres is not a number most people have an intuition for, which is why there is a board at the site doing the work with pictures instead. It sets the chamber's profile against two things a visitor might know: a church tower and the Statue of Liberty.
The comparison that lands hardest for most people is the second one. The chamber is deep enough that the Statue of Liberty, on her pedestal, would stand inside it with room to spare above her torch. That is not a figure of speech about a large cave. It is a measured comparison, drawn to scale, on a sign at the crater.
The other useful comparison is horizontal rather than vertical. The floor is broad enough that a full group spreads out across it and looks small, which is exactly what the photographs of a single figure on the rubble are showing. Those are not wide-angle tricks.
The shaft from the floor, the entrance a bright point far above
The shape of it
Picture a bottle rather than a well. The neck is the shaft you enter through; below it the body of the chamber opens out on all sides. The walls are not smooth and they are not vertical: they lean, they bulge, they carry ledges and overhangs, and in places the rock above you is further out than the rock at your feet.
The floor is not the original floor of the chamber. It is a rubble pile, formed from material that has fallen from the walls and the roof over four thousand years, and it slopes. Walking on it is walking on a scree heap indoors.
There is one more thing worth knowing before you go down, because it changes what you look at. The chamber is not a cave in the ordinary sense. A cave is a space water or lava carved out of existing rock. This is a space that was once entirely full of molten rock, which then left. Everything you are standing in was the inside of the volcano's engine.
What the walls are made of
The rock itself is basalt, which is what almost all Icelandic volcanism produces: a dark, fine-grained lava low in silica and therefore runny when molten. That runniness matters to this story, because it is part of why the magma was able to drain away rather than stiffening in place.
What you actually see, though, is rarely dark. The surfaces are coated and stained by minerals deposited out of hot gas that moved through the rock as it cooled, and those deposits are what give the chamber its reds, yellows and violets. Underneath the colour it is ordinary Icelandic basalt.
In places the wall carries the texture of the melt directly: drips, runs and smooth flowed surfaces where lava was in contact with rock and then withdrew. Those are worth looking for. They are the closest thing in the chamber to a record of the moment it emptied.
120 m
The depth of the descent. The site's own scale board sets the chamber against a church tower and the Statue of Liberty, because the number alone does not land.
How much of it you actually see
Not all of it, and this is worth setting expectations about. The chamber is lit by lamps placed on the walls, and lighting a space this size from a handful of fixed points leaves a great deal in shadow. The upper reaches, in particular, go dark.
What that produces is arguably better than even illumination would be. The lit sections carry the colour, the dark ones give you depth, and the bright point of the entrance hole far above gives the whole thing a scale reference that a floodlit chamber would lose.
Photographs vary enormously as a result, and this explains why the images you have seen do not agree with each other. Different exposures pull different parts of the chamber out of the dark. None of them are lying; they are each showing a slice.
Your eyes adjust over the hour you are down there, and most people report that the chamber seems to grow during the visit. It does not, of course. You are simply seeing more of it.
