(AKA My Little Hellmouth?)
Technically, it's the spring overflow, and as you can see, 2000 years of running water has deposited a lot of mineral salts on top of the original stone. It's quite freaky when you're actually there - it's a hot spring, so there's a lot of steam as well as this gaping orange maw. Hard to capture the steam in photos; I'm rather glad to see the official photo at the link above doesn't show you the steam properly either!
