One of the best choices is PVC plumbing pipe (schedule 40). It's designed to be buried, and with glued end caps, it's also (obviously by design) waterproof. It

One of the best choices is PVC plumbing pipe (schedule 40). It's designed to be buried, and with glued end caps, it's also (obviously by design) waterproof. It has to be cut open, and it obviously wouldn't be good for storing huge amounts of stuff.