My first thought with the term "tank graveyard" is basically a stockpile of mothballed military equipment. This is a real thing that exists in many places, both with tanks and other items.
If I was a player and was told I came across one, the first thing I would investigate is why the tanks (or other equipment) were mothballed. Sometimes it's because they're destroyed (like the post-WWII plane graveyard scenes in The Best Years Of Our Lives). Sometimes they're just decommissioned because there's new equipment. And sometimes tanks are mothballed immediately after manufacturing due to over production. Depending in the situation, a tank graveyard could be a dangerous dump or a playground of free firepower.