You can't bathe in these, but Dallol Sulphur Springs in Ethiopia’s Danakil Depression offers a mesmerising landscape that wouldn’t look out of place in a sci-fi movie. Here, steam is spewed out from fissures in the Earth’s crust and the boiling magma below the surface heats the watery pools. The rocky salt deposits get their striking, neon colours from minerals dissolved within the waters. Visit in November or December, when the searing heat lessens.