Mercury swamp

There were mercury swamps located on the planet Vulcan. They contained rocks and pools of mercury that occasionally sprayed liquid mercury.

A Dalek spaceship once crashed in the mercury swamps of Vulcan. Hundreds of years later, colonists found the ship in the swamps. The Second Doctor, Ben and Polly landed in the swamps and explored. The Doctor found a body. Ben and Polly were overcome by mercury fumes and rescued by the colonists. (DW: The Power of the Daleks)