With a network of channels, lakes, and watery passageways spreading out over several kilometers, Ali Sadr Cave (غار علی صدر) is one of the largest subterranean water complexes in the world. Stable since its formation millions of years ago, the sunless grottoes feature clear waters, fantastical rock formations, and stalactite-covered ceilings soaring high overhead.

Unlike many other aquatic caves, the body of water in Ali Sadr is not a flowing river, but rather an extensive, web-like lake seemingly fed by spring waters from nearby Sarab Cave. Visitors to Ali Sadr are conducted to an underground wharf where they board boats that take them gliding silently through this chain of flooded chambers.

Colorful, glittering geological structures cover the tunnel walls above and below the surface of the water, which is clear enough to provide visibility to a depth of five meters. Tight, winding passageways branch off endlessly from one another, or open up into massive rooms like the Wedding Room (also called the 1000 Stalactites Room), the Island (which sits at the center of the complex), and the 600-meter-long Freedom Hall (the largest chamber in the network).

Prehistoric paintings, pitchers, and jugs found onsite indicates that the cave were inhabited by early humans starting 12,000 years ago. An additional entrance to Ali Sadr was constructed during the reign of Darius I (521-485 BCE), but at some point after that the cave was apparently forgotten, being rediscovered only in 1963. Since then it has been explored by mountaineers and geologists from both Iran and abroad. Its known extent at this point stretches over 11 kilometers, with its longest passage mapped thus far running almost the same distance.

The standard tour takes visitors on a 2100-meter journey through Ali Sadr Cave, 1470 meters of which is covered by boat—making it one of the longest underground boat trips in the world.