Thrihnukagigur

When a volcano stops erupting the magma in its chamber normally solidifies or drains and the roof collapses. At Thrihnukagigur, which last erupted about four thousand years ago, the chamber emptied and the roof held - leaving a cavity roughly a hundred and twenty metres deep and large enough to hold several airliners, with the original vent open at the top. It is the only place on earth where the inside of a magma chamber can be entered. Access is by an open platform lift, adapted from window-cleaning equipment, lowered through the vent on a cable in about six minutes. The rock walls are stained red, ochre and green by minerals. Numbers are strictly limited and the season is short.

What you actually see

• The inside of a magma chamber - the only one on earth open to entry.
• A window-cleaner's lift - the open platform lowered through the vent.
• A chamber 120 metres deep - big enough to hold several aircraft.
• Mineral-stained walls - red, ochre and green across the rock.

When to go

Mid-May to October only. A day.

Honest expectations

It is among the most expensive single excursions in Iceland and books out months ahead. The season is short and tours cancel in bad weather. The approach is forty-five minutes over rough lava in both directions. It is cold and dark inside.

Nearby

Reykjavik is half an hour away; the Blafjoll ski area is alongside.