Krubera Cave
The difference in elevation of the cave's entrance and its deepest explored point is . It became the deepest-known cave in the world in 2001 when the expedition of the Ukrainian Speleological Association reached a depth of which exceeded the depth of the previous deepest-known cave, Lamprechtsofen, in the Austrian Alps, by . In 2004, for the first time in the history of speleology, the Ukrainian Speleological Association expedition reached a depth greater than , and explored the cave to . Ukrainian diver Gennadiy Samokhin extended the cave by diving in the terminal sump to 46 metres' depth in 2007 and then to 52 m in 2012, setting successive world records of 2,191 m and 2,197 m, respectively. Krubera is one of the two known caves on Earth deeper than 2,000 metres.
Ghar Parau
A British led caving expedition to the Zagros Mountains in 1971 led to the discovery of the cave. The cave was explored to a depth of 742 m by the time the expedition was forced to return to the surface. The explorers believed that the cave might go on much further, possibly surpassing the Gouffre de la Pierre-Saint-Martin system, which at that time was the world's deepest known cave.