The Old Mines were the first highly organized and well engineered exploitation of Cerro Rico by Duke Carlos Silverhand, began roughly 200 years ago.
Carlos attempted to bring order and design to the haphazard series of tunnels and shafts that had gone before them, greatly increasing the mine's output and safety.
The Old Mines mostly focused on several rich mithril deposits that lay directly below the peak of the mountain. The are centered on a large horizontal shaft (called the Old Drift) that begins on the mountains shoulders about 1000 feet above the plain. The mine burrows upwards from that shaft following the mithril seams toward the peak. At the very topmost levels, the Old Mine connects to the Labyrinth in several places, more through accident then design.
This placed them well above the water table and provided easy access to fresh air via vertical ventilation shafts, thus greatly simplifying the construction.
Around 50 years ago the mithril veins in the old mine played out and the The New Mine was built, burrowing deep under the mountain. The Old Mine was repurpose to support the operation of the New Mine. It now contains storage areas, machine shops and barracks (some would say prisons) for the workers: