Water and Gas in Caves Additionally, underground rivers find their way into caves through entrance holes. They make the cave bigger through erosion. When an underground river exits a cave, it is called a spring. Apart from the limestone caves, some caves are formed by a hydrogen gas. It comes from deep below the Earth and rises to the surfa The gas dissolves or eats away at the rock and creates a cave other caves are sea caves. They are found along coastlines. The waves pound the coastline, eroding weak areas in the rock