Caves

thumb|Lechuguilla Cave, [[New Mexico, United States]]

300px|thumb|The nest of the Peruvian booby is made of almost pure guano. thumb|Human-made Bird Island (Namibia)|Guano Island near [[Walvis Bay in Namibia]] Guano (Spanish from ) is the accumulated excrement of seabirds or bats. Guano is a highly effective fertiliser due to the high content of nitrogen, phosphate, and potassium, all key nutrients essential for plant growth. Guano was also, to a lesser extent, sought for the production of gunpowder and other explosive materials.

thumb|Caving in a muddy section of Black Chasm Cave in [[California|alt=A man caving in muddy passage with helictite formations on the walls and ceiling]] thumb|220px|upright|Stephens Gap, a vertical cave in Alabama

cave with significant vertical passages

lake mainly located beneath the Earth's surface

thumb|right|300px|Illustration of mine collapse aftermath, from 1878

thumb|''Saracen's tent'' in Luray Caverns in [[Virginia is considered to be one of the most well-formed flowstone draperies in the world]]

protection of caves to minimise the effects of human activities