caldera

Lake Taupō, from which the town takes its name, is a large caldera, a volcano that has collapsed in on itself.

It was in an area overlooking the caldera - the large crater of the volcano - and the boy would not have survived the fall, Ms Ferracane said.

Erebus is only a 20-minute helicopter ride from McMurdo, so since the 1960s scientists have studied the volcano and the lava lake that roils within its caldera, occasionally tossing out “bombs” of molten rock.

The area surrounding the caldera has been closed to the public since 2008 because of other hazards, including ground cracking, instability in the crater wall and rockfalls.

As the reservoir depleted, the ground above it -- the crater within the caldera at the volcano's summit -- suddenly collapsed.