Sump (cave)

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A sump, or siphon, is a passage in a cave that is submerged under water.^[1] A sump may be static, with no inward or outward flow, or active, with continuous through-flow. Static sumps may also be connected underwater to active stream passage. When short in length, a sump may be called a duck, however this can also refer to a section or passage with some (minimal) airspace above the water.

Depending on hydrological factors specific to a cave – such as the sea tide, changes in river flow, or the relationship with the local water table – sumps and ducks may fluctuate in water level and depth (and sometimes in length, due to the shape of adjacent passage).