Large underground cavities are used to store natural gas, which Russia then sells to you Europeans (and us Ukrainians too). Nuclear blasts are the cheapest way to build those, and if the gas is slightly active - who cares?
There was a project to redirect large rivers which now flow into Arctic ocean to flow south, into Caspian sea (where fresh water is scarce). It was planned that a chain of nuclear blasts would create a trench suitable for that project. Test trench (short, only 3 bombs) was even created (there is a lake there now, still noticeably radioactive, but not dangerous), but it was soon found that radioactivity levels would be higher than expected and resulting super-river will have unstable shores.
And as for abandoned mines, I live in the Donetsk coal basin region. We have lots of those. Mines were closed in 1990s-2000s, and now are being flooded by underground waters - not a good thing.