Caverns of Rare Device!
For millions of years, shells of sea creatures collect on the floor of primitive oceans and eventually compact into rock-like limestone. Magnesium converts some of the limestones to slate-colored dolomite. Eons pass and somewhere, violent seismic activity pushes up the ocean floor. If the push is somewhat gentle, only the hills rise above the sea forming dramatic landscapes seen in countries like China, Vietnam, and New Zealand. A famous example is the Ha Long Bay in Vietnam.
