An inland ocean

This majestic swimming hole on Samoa's Upolu Island sits just inland from the Pacific coast. It was formed when the roof of an ancient lava tube collapsed, exposing the 98ft deep pool. This is the largest of many such tide pools and blow holes formed in the area many thousands of years ago. An underground cave system connects this swimming hole to the nearby ocean, and the water rises and falls with the tides.
© Chris McLennan/Alamy