...are right here on Earth. Check this out:
Calcium carbonate spills coming from thermal springs under the mountain have formed a series of spectacular and beautiful pools.
Most science fiction and fantasy writers can't come up with anything anywhere near as awesome or creative as this for their settings. I know I never have, and I can't think of anything I've read that came even close, leaving out the spectacular-engineering type SF books, such as Ringworld. For natural wonders, though, whether set here on Earth or on an alien planet or in a magic-filled fantasy world, nothing can match mundane Nature.
Which isn't to say we shouldn't be trying. :)