The Devils Hole pupfish is often described—probably accurately—as the “rarest fish in the world.” The fish is about an inch long and sapphire blue, with wary-looking dark eyes. It lives in only two places on Earth. One is the real, or at least original, Devils Hole, a pool of exceptionally clear, geothermally heated water found at the bottom of a limestone cavern in the Mojave Desert. The second is the fake Devils Hole, a pool of very clear, artificially heated water found inside a large shedlike building, also in the Mojave. The fake pool has been designed to imitate the uppermost twenty feet of the real one as closely as possible, down to the contours of its sides, which are made not from rock but from plastic foam sealed with fibreglass.

The ersatz Devils Hole was constructed by the United States Fish and Wildlife Service, after the population of pupfish in the real pool dropped to a mere thirty-five individuals, in 2013. The idea was to create a backup population as a stay against the species’s extinction. Several years ago, when I visited both pools, the population of the genuine pool was bigger than that of the simulacrum. Now, however, the situation is reversed: more Devils Hole pupfish live in the mock Devils Hole than in the actual one.