An underground, waterproof cache has many uses from prepper survival to an ingenious geocache to hiding a house key. PVC is a great inexpensive and fast way to hide something underground. It is also often done wrong. I'll walk you the process of getting it right the first time.
Step 1: Materials
Length of PVC pipe - diameter of your choosing
2 PVC Caps
1 Test plug (In PVC pipe section of the hardware store)
Post hole digger
Saw if cutting pipe to length
Silica Gel (from craft store flower section) or Damp Rid refill
Sock or pantyhose.
Don't Buy a screw cap end plug
Step 2: Pop a Cap
Cut PVC pipe to desired length.
Using a liberal amount of PCV cement attach one cap to an end of your pipe.
Let the glue fully cure.
Step 3: Mitigate the Moisture
Unless you are in a desert the ground is a damp place. That temperature variance can cause condensation on the inside of your burial tube. Depending on what is packed and how you pack it you may seriously want to add an additional layer of moisture protection.
Silica gel can be purchased in bulk from online suppliers like amazon or in the flower section of craft stores. Silica gel will trap the excess moisture within itself and is a good solution for long term storage. Pour some in the pipe. If you are concerned it will get into the contents of your stored materials place it in a clean sock or piece of panty hose. Secure the sock or hose with a knot and a zip tie. Silica gel will continue to absorb moisture until it is saturated. It will not absorb any past that point. However it will seek humidity equilibrium. If the moisture content of the air drops below the moisture content of the silica gel the gel will release the moisture back into the air. This is why plant nurseries put it in their soil.
DampRid is a product that is sold to help pull moisture out of the air and trap as well. It is typically used in basements and campers. Much like silica gel packets. Unlike silica gel DampRid will continue to absorb moisture past the point of saturation. Once saturation occurs it will drip down. Place the damprid in a small canister with holes in the top. This will catch any weeping water.
Step 4: Plug It the Right Way
A test plug is a rubber gasket that has a wing nut and a washer. When you tighten the wing nut it pushes the rubber out to the PVC pipe creating an impermeable water barrier. Unlike the more expensive thread type pvc plugs the rubber test plug will not leak. Thread style plugs will allow small amounts of water to weak down the threads and into the contents of your cache.
Insert the contents into the tube and tighten the test plug.
Pro tips: If you are going for long term underground storage remove the wing nut and coat the threads with a liberal amount of automotive grease. This will keep the threads from rusting, corroding or freezing up. It will also allow you to remove the test plug a little easier when you come back to retrieve the contents.
Take your 2nd pipe cap and cover the test plug. This will keep water and dirt off the plug. This will help prevent damage and make the plug wing nut easier to use in the future.
Step 5: Drop It Like It Is Hot
Mark where you want to bury your cache. Make note of physical land marks. Dig the hole and drop in your newly assembled tube. Cover the hole and consider placing a rock, stone, paver, garden gnome, etc on top to make finding it easier for you in the future.