Apparently the Soledar Saltmine is actually quite large.

>The scale of the mines is vast, with 125 mi (201 km) of tunnels,[1] are at a depth of 288 m (945 ft),[2] and many of the chambers are 30 m (98 ft) in height.[3] The largest resembles a hangar of about 100 m (330 ft) length and 40 m (130 ft) width and height, and has accommodated soccer matches and the inflation of a hot air balloon.[4]

https://en.wikipedia.org/wiki/Soledar_Salt_Mine

I wonder how much use this was for the Ukrainians and what it changes now they lost it.

Here is Wagner in the Salt mine, there is an ammo cache seen in this footage as well.