Extreme? PSHH! I've never done any riding difficult enough to even remotely warrant that title!
I bumble around farmland and woods, with nary a cliff to shake a stick at. Plus there is the matter of having a good, surefooted horse. My mentally sound trail horses would run you straight into a tree and trip over a lady bug. My surefooted ones would dump you off at the sight of a turkey feather, and forget going it alone! I hope to get more experience on the spring chickens to the point where they ride like a horse their age should. (My two good mares are 16 and 19, the sassy ones are 6 and 8.)
My idea of extreme would be riding 15 miles of the nice crushed limestone trail that used to be an old railroad track. Although, I must say, I did ride across a wooden bridge that was a few hundred feet above the water. Now THAT was scary, but I was the only one to actually ride across.
Not that I am a bridge expert, but that bridge looked like it could have used a few more support beams. And maybe could have been made out of something more solid than wood a little over an inch thick. I can't imagine myself riding on cliffs, anywhere. I don't know what my horses would think if I walked them up to a rock that was chest-high and asked them to hop on, either.
This thread has given me an idea... Perhaps I should curve off the trail and head up to the ridge for a bit more of a challenge to the poor old mare? Or even sneak in early in the morning and stir up some deer or turkey for a little bit of desensitization..