Dikes, sills, and volcanic pipes are all types of intrusions, a word geologists use to describe places where magma has pushed its way upward through the Earth's crust and cooled close to the surface. Dikes are places where the molten rock has solidified in a sheet that cuts through layers of rock bodies. Sills, on the other hand, are intrusions that squeezed in between existing layers of rock. A volcanic pipe forms when a tube-like conduit of magma to a surface volcano solidifies after the volcano's eruption has ended. Because these large-scale intrusions are formed underground, humans must wait a long time, sometimes millions of years, until erosion, mining, or digging removes the surrounding rock, revealing the deep structures.