"The unique land form known as the Chocolate Hills of Bohol was formed ages ago by the uplift of coral deposits and the action of rainwater and erosion."

"About two million years ago, most of the island of Bohol was below a shallow sea. Coral reefs, similar to those now found offshore of Northern Bohol, thrived and extensively covered the sea floor. During stormy days, fragments of corals and shells derived by waves from the reefs were deposited mostly at the landward side of the reefs. The coral and shell fragments formed relatively thin layers(brown) surrounding live coral reefs(pink).

Slowly, the land rose causing the coral reef formations to emerge out of the sea. The Chocolate hills had been carved out from the relatively thin layers of coral and shell fragments. During the initial stage, gullies were developed in the below lying areas and lakes occupied pre-existing depressions.

Coral and shell fragments are largely composed of calcium carbonate, a chemical compound which can be dissolved by acidic solution. Rainwater becomes slightly acidic by dissolving some carbon dioxide from the atmosphere. Thus, rainwater can dissolve coral and shell fragments. But this happens very slowly, with rainwater acting on the layer of coral and shell fragments for tens to hundreds of thousands of years. Gullies were deepened and widened to become streams, lakes were emptied by underground rivers and interconnected conical hills were formed from the original flat surface.

When the bast of the soluble formation was reached, down-cutting ceased and lateral erosion became dominant, valleys were widened. The remnant of the layers in the highly dissected areas were dissolved away. And the connections between neighboring hills were also dissolved away. This, the chocolate Hills that you now behold are products of the patient laboring of rainwater on a thin soluble limestone formation."