It seems quite a few caches are buried flush with the ground.
Some of the Ebay containers are built specifically for burying.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewI...5762476762&rd=1
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewI...7141980203&rd=1
I suppose if you asked the BLM, they would tell you it is illegal to disturb the rocks or something.
If you aren't in a park, I don't see the harm in digging.