Recently, there have been several letters or articles at Survival Blog on caching your firearms in case they are needed. Obviously, I have no experience in this area, but several readers have written in about tests they have done. Apparently, there are several keys to long term storage of firearms underground. First you must store them properly, to protect them from the moisture. The key with this trick is a completely waterproof container and a backup plan of moisture absorber. When burying, you do not want to bury the tube vertical, it is harder to get out of ground that way. You can also seed the area with scrap metal to hide the signature of the tube. You can also store ammo and other essential gear, because if you are digging up your guns, you will probably need other gear also.
(I will leave the discussion of what kind of situation would necessite such drastic action to another day, but I am sure you will understand that if such drastic action is needed then what I am about to say would be a sane response to insane times.) I had another idea on the long term storage of guns, that is a little more expensive than the military sonar buoys mentioned. What is large, perfectly normal to have one of, and is guaranteed to keep the contents protected like tupperware in a refrigerator. A burial vault (above ground would be best) of a loved one would make a perfect place for long term storage of weapons, ammo, and gear. If you don't have time to prepare such an elaborate scheme you could possibly break into a graveyard and bury a casket underneath a current one in the dead of night or preferably bribe the caretaker to do it for you. The gravestones would make an excellent long term marker to find your cache later.