They Enter The Chamber Of Fire...

Thrihnukagigur is a dead volcano just outside Reykjavik, Iceland. It stopped belching fire 4,000 years ago. No man dared go anywhere near the inferno when it was alive. Dead now, the government has thrown it open to visitors. 

For the first time, lion-hearted adventurers can climb over the exposure, actually a small fissure on the side of the volcano, and descend into the volcanic chamber that was once rut-rutting with molten fire, ready to be ejected any moment.

Cave explorer Arni Stefánsson is believed to be the first to get into the the belly of the volcano. He had gone deep into it at a considerable risk to his life, and found nothing interesting there. That was in 1974. Seventeen years later he entered again, this time with illuminating equipment, and returned super-awed.

Volcanic caves left out as geographical malformations, are rare. Furna do Enxofre in the Azores is one of a type, but not as big and complete as Stefánsson's find.