Mxsmanic wrote:
If there were no airspace restrictions, would it be possible/safe to fly
gliders in the Grand Canyon? There's not much of a place to land in the
canyon, of course, but I don't know if there are thermals or something (?)
that would allow a glider to climb back up out of the canyon after gliding
below the rim. Is it possible? Has anyone done it?

Go read SFAR 50-2 and come back if you have further questions.
You can't operate even a powered aircraft below the rim in most
places.