This information is from Albion, Ector as of 08/20/2005.
While killing mobs in Abandoned Mines you may receive an item called a Tarnished Supply Chest Key. If you delve it, it says that it opens an Old Creaky Supply Chest, somewhere in the Abandoned Mines. The one named mob I can remember that dropped the key was Lur Tai, but it dropped from other regular mobs as well, mostly tenebrae as I recall.
The chest is actually named Old Rickety Supply Chest. It is located at 9197, 30191, 17633. To find it, start from near the entrance to Stonehenge Barrows in Abandoned Mines. Go south, and at the first fork in the path turn West, follow that tunnel as it turns South. Take the second West opening you come to. It is a short tunnel (more like an alcove). The chest is in the back. There are several Greyfur gnolls around the chest, which conned grey to me at level 40.
If you are carrying the key when you right click on the chest, you will receive randomly generated magic items. Of the six items I received from the keys I had collected, all six were accessories, including jewelry, cloaks, and belts. They ranged in level from 26 to 29, with most being level 29.