The Jeita Grotto is a system of two separate, but interconnected, karstic limestone caves spanning an overall length of nearly 9 kilometres (5.6 mi). The caves are situated in the Nahr al-Kalb valley within the locality of Jeita, 18 kilometres (11 mi) north of the Lebanese capital Beirut. Though inhabited in prehistoric times, the lower cave was not rediscovered until 1836 by Reverend William Thomson; it can only be visited by boat since it channels an underground river that provides fresh drinking water to more than a million Lebanese.
In 1958, Lebanese speleologists discovered the upper galleries 60 metres (200 ft) above the lower cave which have been accommodated with an access tunnel and a series of walkways to enable tourists safe access without disturbing the natural landscape. The upper galleries house the World's largest known stalactite. The galleries are composed of a series of chambers the largest of which peaks at a height of 120 metres (390 ft).
Aside from being a Lebanese national symbol and a top tourist destination, the Jeita grotto plays an important social, economic and cultural role and is a finalist in the New 7 Wonders of Nature competition, and as of 7:44 pm GMT the provisional New7Wonders of Nature based on the first count of vote results on 11/11/11 Jeita was one of the top 14 Finalists in the New7Wonders of Nature.
The Jeita grotto is located within the Lower-Middle Jurassic strata of Keserouane which has a stratigraphic thickness of 1,000 metres (3,300 ft) and consists of dolostone and micritic limestone. The Keserouane formation was exposed to air by a local uplift during the Late Jurassic-Early Cretaceous. The Keserouane strata became karstified after its aerial exposure and then was buried during the Cretaceous. In the Neogene, upon the final uplift of Mount Lebanon, this early karstification phase was reactivated.
At the Nahr al-Kalb valley, the impervious Upper Jurassic volcanic rocks and Lower Cretaceous sand slant almost vertically forming a hydrogeological barrier and forcing the outlet of the Jeita underground river to the surface. This barrier could be the reason for the westernmost, large cave chambers with heights exceeding 60 metres (200 ft). Karstification of the Keserouane limestone was further intensified by the steep topography of the area and the volume of precipitation over the Lebanon (more than 1200mm).
The Jeita upper cave has an overall length of 2,130 metres (6,990 ft) of which only 750 metres (2,460 ft) are accessible to visitors via a specially conceived walkway; access to the remainder of the cave was restricted to prevent ecological damage which may occur due to the flocking tourists. The upper cave contains a great concentration of a variety of crystallized formations such as stalactites, stalagmites, columns, mushrooms, ponds, curtains and draperies.
The upper gallery is famous for its formations, lit by an effective lighting system. It is entered through a 117 metres (384 ft) long concrete tunnel. The part accessible by visitors has three huge chambers. The first is called White Chamber, the second Red Chamber, due to the color of the formations. White dripstones are pure calcite without defilement, the red color is given by iron oxide (rust) in small amounts. In Lebanon iron oxide has a red color instead of the brown beige color which is common in northern countries.
The reason is a different chemical reaction caused by the high temperature which produces a different kind of iron oxide. The White Chamber is medium sized, but has the most impressive formations of the cave. The Red Chamber is up to 106 metres (348 ft) high, and 30 metres (98 ft) to 50 metres (160 ft) wide. The third chamber is the biggest of all three chambers and has a height of more than 120 metres (390 ft). The longest stalactite in the world is located in Jeita's White Chamber; it measures 8.2 metres (27 ft) long.
The lower gallery which has an overall length of 6,200 metres (20,300 ft) is located 60 metres (200 ft) below the upper gallery. It is traversed by a smooth underwater river and a lake (the "Dark Lake"). The river is broken up by several small cataracts and rapids. The lower cave's "Thompson's Cavern", is a massive hall with impressive speleothems such as the Eagle Obelisk stalagmite. Other halls in the lower gallery include the Pantheon, Grand Chaos and Shangri-la. Visitors are transported in the lower gallery by electric boats for a distance of 500 metres (1,600 ft). In winter the lower level is closed, when the water level is too high.