Sima Humboldt (Sima Mayor) is an enormous sinkhole located on the summit of the plateau of Sarisariñama tepui in Bolívar State, Venezuela. It is unusual for several reasons, including its enormous size and depth, its location on the top of the only forested tepui, having a patch of forest on its base, and also due to the weathering process that formed this sinkhole. The feature is named after scientist and explorer Alexander von Humboldt.

Together with the neighboring Sima Martel, it was first spotted in 1961 by pilot Harry Gibson. Only 700 meters from the rim of Sima Humboldt there is another enormous sinkhole, Sima Martel. In total there are four caves in Sarisariñama.

The sinkhole was descended for the first time in 1974 and more thoroughly explored in 1976. Its volume is 18,000,000 m3, with a maximum width of 352 mat on its upper rim and 502 m below, and a depth of 314 m.

At the bottom of Sima Humboldt is a completely isolated forest. Sinkhole openings where mini-jungles have grown are known as dolines, and they are filled with life, which has been buffered from the outside world by the cave.

According to the Wikipedia/ livescience