[fghorow]

It will be interesting to see how much net energy is able to be stored after the energy cost of de-watering the mine is taken into account.

(An interesting tidbit of trivia I learned from miners is that deep underground mines only became feasible after reliable pumping was put in place.[1] Prior to that, they flooded out from groundwater intrusion.)

[1] https://en.wikipedia.org/wiki/Flooded_mine

Edited to add the wikipedia link.

[defrost]

Depends on base depth and water table - It's 1.4 km "deep" from the head .. which might be 2.5 km above sea level in Finland (what with all the steep bits and everything).

If the bottom is well above sea level and not in line of any underground rivers | streams | lakes it'll stay relatively dry.

For interest:

https://en.wikipedia.org/wiki/Pyh%C3%A4salmi_Mine

    The "main level" of the mine is at 1400 meters depth and can be accessed with either the mine hoist or via the access tunnel. The main level houses a cafeteria, washrooms, showers, workshops, storage facilities, as well as a safety area.

    It is also home to the world's deepest sauna, at 1,410 metres (4,626 ft) underground.

    Pyhäsalmi Mine has hosted numerous events due to its attraction as a unique location.

    It has hosted the deepest concert in the world (by Agonizer at 1271 m ) as well as dance performances.

    The 11 km long spiral-shaped main tunnel has also seen several uphill running and cycling competitions. 

    Pyhäsalmi Mine can be recognized as a filming location for the new sci-fi television series White Wall, premiered in 2020. 

Ahhhh, Finland, Finland, Finland.

[helsinkiandrew]

>which might be 2.5 km above sea level in Finland (what with all the steep bits and everything).

Steep bits? Are you thinking of Norway :-) there’s no steep bits in Finland, highest point above sea level is 1324M!

There was even a Norwegian campaign to gift a mountain for its birthday https://www.bbc.co.uk/news/world-europe-37662811

[defrost]

The bloody cheek of those Norwegians!!

It's all relative - I live in a state 3x bigger than Texas and the highest point is only 1248m .. when I visited (man years ago) lot's of places in Finland looked steeper than here.

[kitd]

Mine flooding was the driving force behind the development of the original steam engines.

[beembeem]

One solution, you schedule pumping during times when the battery is charging, because presumably there is far more power available than can be used to charge the gravity battery anyways.