The military uses vented bunkers with dirt over them. A lot of that is an attempt to moderate the temperature and keep things cool in the summer and warmer in the winter.
Most military ammo is cycled out of service either by firing it or by determining it's aged too much and has too many misfires. There might be old WWII surplus but duty ammo that old is rare - entirely why they sold it off.
Moisture is the #1 problem - and why military packs are in metal ammo cans sealed with gaskets with dessicant packs inside. Brass will corrode in contact with moisture - and air with moisture gets everywhere. If it's warm humid air in contact with colder metal, moisture will condense and build up. A bad seal on an ammo box will actually cause the box to fill with water in extreme conditions, like the trunk of a car. Hot weather will expand the air driving it out, cold weather will cause it to suck in more air to deposit more moisture.
Ammo is NOT water proof, the military seals the primers and the necks are crimped.
While everyone says ammo doesn't need to be stored in climate controlled conditions, what they miss is that ammo does very much need to be stored in humidity controlled areas where it won't have condensation build up wetting the brass or the box it's stored it - which will get damp from absorbing the moisture and keep it in close proximity. Goes to those sealed metal ammo cans with rubber gaskets in the military. Each one is a dry controlled micro chamber.
If you want to shoot it ten years later store it dry and keep it dry. Using surplus metal cans may mean using a food grade silicone grease on the seal to prevent leaks - that can was sold off for a reason.
It very much is about humidity control, and the military spends a lot of money on it. Once bundled the crates are palletized and they are now stretch wrapped for storage in bunkers. There is a lot done to keep ammo dry - larger rounds use individual containers which are sealed against humidity and for protection in shipping.
Keep your powder dry.