These “reservoirs” of the Earth’s heat—geothermal ﬁelds—lie several miles below the surface in areas of geologically recent volcanic activity. The heat is generated by the decay of radioactive substances in rocks lying even farther below in the Earth’s crust, and from compression as the rock becomes deeply buried by geological processes. The land in and around Yellowstone contains such large ﬁelds, as evidenced by the numerous “surface thermal displays” of hot springs and geysers, spectacular but merely visible signs of the much deeper, bigger, and more complex ecosystem below. Yellowstone’s geothermal wonders were the original justiﬁcation for the park’s creation in 1872, and the area is now legally deﬁned as a geothermal resource. Of the major geyser areas of the world, only two—Yellowstone and the Kamchatka Peninsula of extreme eastern Russia—have not been massively disrupted by energy development. Many in the mining industry contend that the exploitation of geothermal ﬁelds would provide clean energy, but fail to fully address the many ecological problems—not the least being the radioactive contamination of rivers and streams—that would result.