Plutons are igneous rocks that in their magma form were intruded into pre-existing rocks and then cooled slowly, all beneath Earth's surface. Because the cooling period is so slow, plutonic rocks have time to form visible crystals, which can grow quite large. The sizes and shapes of plutons have specific names, including batholiths (over 40 square miles) and boss stocks (smaller than about 40 square miles). Plutons become visible on the surface through erosion. The name "pluton" is a reference to Pluto, the Roman god of the Underworld.