a hollow place in the ground, specifically a natural underground space large enough for a human to enter
a line that forms the boundary between the land and the ocean or a lake
These form where the rock below the land surface is limestone or other carbonate rock, salt beds, or in other soluble rocks, such as gypsum, that can be dissolved naturally by circulating ground water. The ground above the empty space collapses into the hole.
begin high up in the mountains in bowl-shaped hollows called cirques
continuous masses of ice---- called ice fields or ice sheets.
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