You could be forgiven for thinking that the otherworldly rock formations in these pictures come from another planet, or from a sci-fi movie perhaps. But they're actually called sand tufas and they come from right here on earth.

A tufa is a peculiar form of calcium carbonate, and they're created when calcium-bearing freshwater springs well up through alkaline lake water (which is rich in carbonates). The carbonate and calcium combine and, over centuries, these elements form unusual spires, towering columns, and strange cauliflower-shaped mounds which can reach heights of up to nine metres (30ft). Tufas can only form underwater, and they're only exposed when lakes are drained or dry out over many years. Check out this collection of pictures compiled by Bored Panda to see these curious formations for yourself.