It’s an ammo box. Presumably padlocked. Time for a set of bolt cutters.
Everyone’s assuming they’re steel ammo cans.
Is this indeed the case? If so, then it’s likely that whatever’s in them is in pretty good shape, except for temperature swings. ISTR reading that some guy’s war medals and memorabilia were in one and got washed away during Hurricane Ike, ending up tens of miles away, but were none the worse for wear when they were found some time later.
As a PSA, I’ll note that steel ammo cans can be dangerous outside of ammo/explosive risk. There have been cases of folks getting injured, including lost fingers, opening ammo cans that had been used to store pressurized containers. Spray paint, gun cleaning solvent, etc… if the ammo can seal is still good, but a leaky spray can was inside, the whole ammo can can be pressurized. And when you pop the latch, the lid can fly open with considerable force. Wear gloves and pop the latch without your fingers being inside the loop is safest with an unknown ammo can.
Id be interested in a link to a couple of those stories. Then again many common things are dangerous if used improperly: knives, fireworks, hammers, saws, cars…
Nitroglycerin is used in most US small arms ball propellant; not the primer. Not blended in as an additive but as a surface [for better word] spray. Causes the nitrocellulose surface to form cracks. Improves ignition to assure a quick clean burn.
US military military specification primers have been using lead azide as the primary explosive for the last 50 years at least. [psst - don’t trust wiki on this one]. There has been some initiative to switch to a non-lead based primer explosive but there have been some problems.
Given there’s still plenty of perfectly shootable WWII - and even WWI - vintage ammo about, I wouldn’t be calling out the bomb squad for old ammo, personally.
What markings are on the boxes?
The good news for the OP is that if they are indeed screwed/bolted into the ground, they are unlikely to be pressurized.
I couldn’t find any recent news. It was quite some time ago, possibly 20 years, there were a couple newsworthy accidents around the same time, including some guy that lost a finger. I remember a few waves in surplus/gun show markets about starting to demil used ammo cans, and the Army put out a safety flyer saying not to store spray cans in empty ammo cans.
I have had the habit of opening unknown provenance ammo cans from underneath, not putting my fingers in the way, ever since then though.
I will feel a lot better when the OP does come back ! I wonder if someone stored their will and money in the boxes .