Travelling to the Earth's center is a popular theme in science fiction. Some subterranean fiction involves traveling to the Earth's center and finding either a hollow Earth or Earth's molten core. Planetary scientist David J. Stevenson suggested sending a probe to the core as a thought experiment.[1][2] Humans have drilled over 12 kilometers (about 8 miles) in the Sakhalin-I project.[3] In terms of depth below the surface, the Kola Superdeep Borehole SG-3 retains the world record at 12,262 metres (40,230 ft) in 1989 and still is the deepest artificial point on Earth.[4]