I had that same idea a few decades back growing up near one of the largest open pit copper mines in the world. It’s about 1 cubic km in volume. The mine can’t produce forever and the surrounding bedrock and extracted overburden are pretty toxic with heavy metals. Reclamation is going to be tough. I came up with the idea back when there was that NTC garbage barge that couldn’t find a destination that would take it. $10 / cubic meter would be attractive to most cities. Moe if you can take contaminated stuff. The place already had trains, railcars, trucks, etc. The ore produces $5 per ton (not including overburden) before mining and smelting costs. Filling the hole can’t be any more expensive and could be more profitable.
So haul in the the nastiest garbage, use the overburden to cap the layers, and you have at least half a cubic km of space. That’s a minimum of $5 billion in revenue from what was previously a toxic hole.
