New discoveries and new experimental techniques are beginning to open an unexpected window on the hidden depths and workings of the Earth. The Earth's deepest rocks that can be held in the hand were delivered to us 100 million years ago by explosive eruptions of diamond-bearing kimberlite magma. For many years, the depth of wall-rocks carried to the surface has been estimated at about 150 km. Some nodules of the rock known as eclogite, however, may have come from at least 300 km, and perhaps more than 400 km, deep. If the latter figure is correct, the rocks could be samples from the enigmatic transition zone that is defined by changes in earthquake wave velocities, and whose origin is the subject of much debate.