Many diamonds are carried to Earth’s surface from hundreds of kilometers below, where they form, by intrusive magma punching up through the surface in pipelike volcanic structures. The kimberlite is removed to get the diamonds in open-pit mining operations, after which a large hole remains, such as the famous Kimberley mine in South Africa, shown here. The hole fills with water after the mining operation has ceased. Credit: Adrian Marks via Pixabay