Penang National Park

This guide to Penang National Park follows a two-hour trek through hill forest and mangrove that opens onto a beach holding a lake that seems to defy physics, fresh water sitting on top of salt water, refusing to mix. It explains the halocline that separates the two layers, and how rare lakes like it are anywhere on Earth. Only one hardy species of snail is known to survive in the lake's airless lower layer.