Tuno writes:

As a cross-country soaring enthusiast, the notion of soaring below the
rim and out again (assuming the legalities of course) is exciting --
and foolish.

A fundamental rule of safe soaring is that you always have a safe
place to land, iow never assume there is a thermal between you and the
closest place on the ground that will not break your glider. I've seen
the Big Ditch a number of times from the air (and twice from my
sailplane) and have never seen a safe place to land.

Understood.

If you _did_ have a safe place to land, somewhere, would there be enough
rising air in the canyon to safely lift you out after gliding some distance
below the rim? If so, how low could you go? I don't know much about sources
of rising air for gliding, so the question may be naïve.