Multiple choice general knowledge science & technology Diamonds form: at the bottom of certain parts of the Indian Ocean in certain types of caves just below the Earth's surface about 100 miles below the Earth's surface, in the mantle's molten rock bottom of the Pacific ocean Reveal answer Fill a bubble to check yourself C Correct answer Explanation Diamonds form approximately 100 miles below the Earth's surface in the mantle's molten rock under extreme pressure and temperature conditions. They are brought to the surface through volcanic pipes and kimberlite eruptions.