That, looks like a bunker.

Very close there is a large monument tower with fancy restaurant and sight seeing windows; it's called Puijo Peak, build in 60's. This has to be connected into that as sort of ventilation shack/bunker

In general, there are population bunkers build for possible air strikes inside ground stone layer during world word II, but there's none right on this spot. It does give the war bunker look - My home region is far from Russian borders and away from real major historical conflicts really

absolutely love

Ooh, what is the history of that area? That structure looks downright solid!

Very close there is a large monument tower with fancy restaurant and sight seeing windows; it's called Puijo Peak, build in 60's. This has to be connected into that as sort of ventilation shack/bunker

In general, there are population bunkers build for possible air strikes inside ground stone layer during world word II, but there's none right on this spot. It does give the war bunker look - My home region is far from Russian borders and away from real major historical conflicts really

I always love learning the reasons for the architecture of a given area. Myself I have many such artifacts of the Cold War and of past military installations around where I live. There's an old Radar Base near me that is now only visible as what looks like a giant crop circle but at one time was a massive radar array. I've walked the site and you'd never know what was there so many years ago.