Explorers have confirmed the discovery of the world's longest underground river in the Yucatan jungle in Mexico. Divers Stephen Bogaerts and Robbie Schmittner say the river runs an amazing 153 kilometers through spectacular limestone caverns and would span ten kilometers of land if laid down in a straight line. The two divers spent four years exploring the river which they entered through a cenote or sinkhole, which is like a well.
In the Philippines, the most popular underground river is in the St. Paul Subterranean River National park in Puerto Princesa in Palawan. It extends to about 8.2 kilometers. Below is a photograph of its mouth from http://www.virtualtourist.com/travel/Asia/Philippines/Province_of_Palawan/Things_To_Do-Province_of_Palawan-BR-1.html