In the Tom Scott video linked upthread, they cover this. The pit is being used as a hydraulic control device-collecting all the badness from a ridiculously vast

In the Tom Scott video linked upthread, they cover this. The pit is being used as a hydraulic control device-collecting all the badness from a ridiculously vast (~10,000 miles) network of tunnels into one huge dilute volume. Then, they pipe it out to a water treatment plant and then back to the pit. They’ve been able to raise the pH from mid-2 to mid-4 this way.