unknowns at play here This sounds like a fun thought exercise. No idea who we are protecting against but some factors might be: - Moisture. The containers n

unknowns at play here This sounds like a fun thought exercise. No idea who we are protecting against but some factors might be: - Moisture. The containers need to be completely sealed from moisture and have resistance to acid from the soil. - Metal detector. Perhaps this could be mitigated using a skid-steer with an auger drill attachment? Some of the larger skid-steers can use an 80 inch bit. Excavators can use even larger bits and drill at angles to get under solid objects. Rock auger bits can get through some rocks. Layers of gravel might obscure detection from ground penetrating radar. - Location should be obfuscated by trees to avoid satellite and aircraft imaging from Keyhole/Google/Others. - Accessibility. If there is a few feed of rocks above the container and a rope passes through the rocks, then retrieval is less likely to damage the container and one would not need dig as deep for extraction. - Drill numerous holes and find some other use for them whilst singing One of these things is not like the other