Sistema Sac Actun in the context of Sistema Ox Bel Ha

A subterranean river (also known as an underground river) is a river or watercourse that runs wholly or partly beneath the ground, one where the riverbed does not represent the surface of the Earth. It is distinct from an aquifer, which may flow like a river but is contained within a permeable layer of rock or other unconsolidated materials. A river flowing below ground level in an open gorge is not classed as subterranean.

Some natural rivers may be entirely subterranean, collecting in and flowing through cave systems. In karst topography, rivers that originate above ground can disappear into sinkholes, continuing underground until they reappear on the surface downstream, possibly having merged with other subterranean rivers. The longest subterranean river in the world is the Sistema Sac Actun cave system in Mexico.

The Optymistychna (Ukrainian: Оптимістична, lit. 'optimistic', also known as Peshchera Optimistitscheskaya) is a gypsum cave located near the Ukrainian village of Korolivka, Chortkiv Raion, Ternopil Oblast. Approximately 264 km (164 mi) of passageways have been mapped within. As a result, it is the longest cave in Europe and the seventh-longest cave in the world, after Mammoth Cave, Sistema Sac Actun, Jewel Cave, Shuanghedong and Sistema Ox Bel Ha. It is also the longest gypsum cave in the world.

Dos Ojos (officially Spanish: Sistema Dos Ojos, lit. 'Two Eyes System') is part of a flooded cave system located north of Tulum, on the Caribbean coast of the Yucatán Peninsula, in the state of Quintana Roo, Mexico. The exploration of Dos Ojos began in 1987 and still continues. The surveyed extent of the cave system is 82 kilometers (51 mi) and there are 28 known sinkhole entrances, which are locally called cenotes. In January 2018, a connection was found between Sistema Dos Ojos and Sistema Sac Actun. The smaller Dos Ojos became a part of Sac Actun, making the Sistema Sac Actun the longest known underwater cave system in the world.

Dos Ojos lies north of the rest of the Sac Actun cave system. As a separate system, Dos Ojos remained in the top ten, if not the top three, longest underwater cave systems in the world since the late 1980s. Dos Ojos contains the deepest known cave passage in Quintana Roo with 119.1 meters (391 ft) of depth located at "The Pit" discovered in 1996 by cave explorers who came all the way from the main entrance some 1,500 meters (4,900 ft) away. The deep passages include the "Wakulla Room", the "Beyond Main Base (BMB) passage", "Jill's room" and "The Next Generation passage". In August 2012 Dos Ojos was connected through a dry passage to Sistema Sac Actun. With March 2014 the total length of the combined system measures 319.05 kilometers (198.25 mi).