I'm obviously not an expert since I'm the airhead a few comments above who wouldn't recognize a bomb in the ground. But 80 years is plenty to cover an object. Lots of things can happen. A couple floods or rainy seasons will soften the soil. Debris will pile up and rot over it. Animals may step on it and push it even further, especially if the soil is damp.

My parents did some renovations a couple of years ago and left a large pile of old tiles and bricks in the backyard. The pile got reclaimed by grass damn fast and is half buried now. A small object over decades ? It disappears