One of Iceland's most popular attractions is caving.Exploring Iceland's unusual lava caves, most of which formed more than 10,000 years ago, requires only basic caving knowledge and equipment.Ice caves, however, are more challenging and require special clothes and hiking tools.The best-known ice caves are in Vatnajokull- a layer of ice which, at 8,000 aquare kilometers (3,000 square miles), is Iceland's-and Europe's-largest glacier.It also happens to be situated just above an active volcano!