Geologists think that the specific shape of the hills is caused by the influences of the weather during millions of years. The breaking down of the upper layers of the limestone formations, followed by the erosion processes, resulted in these remnants in the shape of cones. In the rainy season the Chocolate Hills are green. In the dry season of each year, the vegetation on the hills gives the landscape a brown colored view, a reason to call them "Chocolate Hills". Across lowland coastal plains and mountainous interiors, the islands of The Philippines have the richest biodiversity on earth. There are 510 species of mammals, birds, frogs and lizards that are only found in the Philippines. In comparison, Brazil has 725 unique species but it is 28 times bigger. But like many countries, this biodiversity is under threat. People are clearing natural habitats to make way for new roads and settlements, and to use natural resources like timber and minerals. In the 1950s, three quarters of The Philippines was covered by primary forest. Today, forest cover has dwindled to only a third of the land. The Chocolate Hills are one of the more unusual landforms in The Philippines. These egg-shaped hills on Bohol island get their nickname from their parched, brown colour during the dry season. It is likely that they were once limestone deposits beneath the sea, uplifted by the movement of plates and then smoothed by wind and rainwater erosion.