Barry Letts in the Underworld

A wholly genuine, absurdly non-Photoshopped photo of someone exploring the Cave of Crystals, Mexico.

Nice to know that sometimes, nature can do CSO that's even more unconvincing than "Planet of the Spiders".

Now look at the photo accompanying the National Geographic article: http://ngm.nationalgeographic.com/2008/11/crystal-giants/shea-text. And try to work out why nothing in modern SF looks quite so improbably alien, when this is just 300 yards under the planet you're on.