caldera

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.

During the experiments, the optical sensor is kept in a controlled laboratory environment in a building that sits on top of a volcano caldera -- a large depression formed when a volcano erupts and collapses.

A caldera remnant -- the remains of a collapsed volcanic crater once host to a lava lake -- can be seen near the center of the structure.

It is likely that the particles were deposited as sediment in the body of water of a "caldera" -- a large cauldron-shaped hollow that forms after volcanic activity.