The title for the world's longest underground river belongs to Sistema Sac Actun in the Yucatán Peninsula of Mexico. Spanning over 378 kilometers (235 miles) of surveyed underwater tunnels, it is a massive freshwater labyrinth that connects numerous "cenotes" (sinkholes). Geologically, it was formed over millions of years as acidic rainwater dissolved the porous limestone bedrock. It surpassed the Philippines' Puerto Princesa Subterranean River (which is 8.2 km long) for the title back in 2007. A major discovery in 2018 revealed that Sac Actun is connected to the neighboring Dos Ojos system, further cementing its status as the longest known system. Not only is it a natural wonder, but it is also a vital archaeological site, containing ancient Mayan artifacts and prehistoric human remains preserved in the crystal-clear water.
The title of “longest underground river in the world” is debated and depends heavily on definitions. However, the most widely accepted claimant is:
Sac Actun System (Mexico)
Located in the Yucatán Peninsula, this is part of an immense flooded cave system.
- Length: Approximately 376 km (234 miles) of mapped underwater passages (as of 2022, and it continues to connect with other systems).
- Why it’s considered a river: While it is a cave system, it contains a powerful, flowing current of fresh water over a saline layer, effectively functioning as a subterranean river. It flows inland from the Caribbean Sea.
Important Caveats (pun intended)
The “longest underground river” is tricky to define because:
- Underground River vs. Cave System: Many long “rivers” are actually water-filled cave passages. True “rivers” that flow in open channels underground (like the Puerto Princesa Underground River in the Philippines) are much shorter.
- Exploration is Ongoing: New connections between cave systems are constantly being discovered, especially in the Yucatán. Sac Actun itself has merged with other systems (like Dos Ojos) to form a single network.
- Alternative Claimants:
- Hamza River (Brazil): Reported in 2011, this is a hypothesized extremely slow-moving flow of water through porous rock deep beneath the Amazon River. It is not a tunnel or cave river, and its existence as a discrete “river” is debated by scientists. If accepted, it would be longer in distance but is a fundamentally different phenomenon.
- Cave Systems in Ukraine & Slovenia: Some very long cave systems have significant underground rivers within them (e.g., Optimisticeskaja Cave), but they are often part of a complex network rather than a single river.
Summary
- For a traditional, explorable, flowing underground river within a cave, the Sac Actun System in Mexico is the longest confirmed.
- The Puerto Princesa Underground River in the Philippines (about 8.2 km navigable) is famous as a UNESCO site and is often called one of the longest navigable underground rivers.
- The Hamza “River” is a controversial and very different geological claim.
So, if you mean a river you could theoretically swim or dive through, the answer is the Sac Actun cave system in Mexico.