[usrusr]

That's what GP suggested. But unless the mine happens to be in particularly water-friendly rock or is carefully prepared for safe repeated flooding/dewatering, that would only speed up the conversion from mine to uncontrolled sinkhole. Many given up mines are expected to have active dewatering running forever, because the cost of letting them collapse or for safe filling would be so much greater than the cost of continued dewatering for the next couple of generations.

For small capacities, using water as the ballast medium would certainly be cheaper. But there's a break-even point in capacity beyond which the cost for readying the volume for water would be higher than the premium you'd have to pay for handling dry mass instead of liquid.