The second longest underground river in the world runs 8.2 km through a cave system of at least 32, and you can take a boat 4.3 km in from the sea. Because the mouth is tidal, salt water pushes up to about 6 km inside and sits under the fresh, with the freshwater layer at the top only a few centimetres thick, so you are floating on a thin skin of river over seawater in the dark. In 2010 explorers found a second floor above the river with its own waterfalls and a cave dome 300 metres up. UNESCO listed it in 1999, it was voted one of the New 7 Wonders of Nature in 2012 and became a Ramsar site the same year. You reach it by road to the coastal village of Sabang and then by boat; permits are capped daily, so book ahead. Palawan.
Palawan, Philippines
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