Jeita Grotto is a cave on two levels in the Nahr al-Kalb valley of Lebanon, twenty kilometres north of Beirut, and the river you are floated along inside it is the water the capital drinks. The lower gallery is done by boat. A small electric one carries you 500 metres along the river, sound behaves oddly under the dome, the lit stone repeats itself upside down in water like black glass, and the passage keeps folding away into the dark ahead. Winter is when that ends: rain and snowmelt off Mount Lebanon raise the river, and the boat run closes until the level drops. The upper gallery is dry and always open. A raised walkway runs 750 metres into it, past pillars where a stalactite and the stalagmite beneath it have grown into one column, to the White Chamber and a stalactite 8.2 metres long. Brochures call it the longest in the world. It is not; a 28-metre spike in Brazil holds that record, and this one is merely enormous, in a room you can walk into.
Cameras and phones are handed in at the entrance, and the reason is the air. Humidity in here sits near 100 per cent, each person in the chamber radiates heat like a light bulb, and the carbon dioxide people breathe out dissolves into the film of water on the calcite and eats at it. Lamps left on long enough grow algae that stain the stone green for good. So this is a gallery you carry out in your head. Getting to the mouth is its own short journey: a toy-sized train up through the gardens of the valley, then a cable car for the last stretch. The temperature falls the moment you step inside. In 1873 engineers from the Beirut water company cut their names into a pillar 625 metres in, which is still called Maxwell's Column. Two hundred metres further on they sealed an account of the expedition in a bottle and stood it on a stalagmite. The dripping lime has been gluing it to the stone ever since.