Detailed studies of the surface indicate that these were formed by a lake caused by the flow of underground water that periodically made its way to the bottom of the crater.

FORBES: NASA Finds Remains Of Ancient Martian Lake

The Ganges was blocked by a mammoth landslide last month, and a big lake formed behind the landfall.

ECONOMIST: From the archive

Cesar Castellanos, the popular mayor of Tegucigalpa, the Honduran capital, gave warning that a temporary lake formed above the city by debris in the Choluteca river might flood the city centre.

ECONOMIST: After Mitch

The CVO reports that a bubbling lake has formed near the vents where the ash and steam has been coming out.

BBC: Mount St Helens spews out steam and ash on 5 October 2004

The lake bed is formed of a fine-grained mudstone, rich in clay.

NEWYORKER: The Martian Chroniclers