Above: A very visible volcano on Earth. We're lucky our volcanoes aren't nearly as sneaky as the Martian ones....Invisible volcanoes, like flying sharks, sound like a bad idea to me. But nature doesn't care what I think... Olympus Mons, the biggest volcano in the solar system, stands 22 km high - but if you stood on its slopes you'd need special equipment to know it was there. Why? Because it's over 500 kilometres across - bigger than Italy. That makes its slopes so shallow that, locally, they would be lost amongst the terrain. And almost all of it stretches beyond the horizon wherever you might stand on it. It’s literally so big you couldn't tell it was there.... You've read our science facts - have you tried our science fiction? You can pick up our book on Kindle, or as a paperback, by following this link......or, if you'd like to help us do things like podcasts, animations, and videos here why not...Above: The pancake-like Olympus Mons.