They are all over the map, with rocky borders on both sides of them.

Has anyone ever fly low over them, about 5 m up, at high speed through them to test your flying skills?

See how fast you can fly through them nap of the earth and turn and navigate without crashing?

They are also good places to hide from enemy helicopters and fighter jets.

Very good fun it is, too, although I wonder why the deserts of Yemen are so well irrigated!

Best height have managed is probably about 10m though, the Hind's slightly slow response of the thing does make NOE flying a bit hairy.

I did it with the Jolly Green Giant and MI-6 Hook and crashed a few times.

I have used these for shelter (and being sneaky) on occasion with varying degrees of success though usually the rest of the flight group don't like to do this and being the last one to hide has never been much of a survival characteristic.
Good on a Free Flight though and it's always a pleasant surprise when/if the rest of your group shows up.

I was so sure I had a list of various canyons on different maps that are good for this but I can't find it! Or it was my imagination of 'what I meant to do' since I do have numerous scribbled coordinates (not always with a map name) and not always with a note of why I wrote them down - a couple are definitely for 'road-skimming' though.