Austria boasts the largest ice caves in the world

Werfen Ice Caves, a network of caves which descend some 40 kilometres into the mountainside, are the largest of their type in the world.

Guided tours of the geological phenomenon are available, but even in the heat of summer, the temperature inside the caves is at an average of zero degrees. As a result, heavy shoes and warm clothing are recommended.

The tours themselves last around 75 minutes and descend one kilometre into the network. Visitors are also required to climb some 134 metres to reach the Ice Palace - around 400 meters below the surface.

According to scientists, the first cracks in the limestone mountain, which would eventually lead to the caves' formation, began to appear around 100 million years ago.

Ice inside the caves is caused by cold air entering the network during the winter and then in spring, water trickling in is frozen by the winter air.