What's Under There???

As I wander from the car park to the library where I work, taking the shortest route across the grass I pass by this strange hatch in the grass.
What is it? What does it hide? What’s under there?
It’s not alone. There are at least two other hatches, one nearby and one in the town park a bit further down the hill.  Manhole covers? No, in Sweden manhole covers are pretty much like manhole covers anywhere, heavy cast iron circles, not wooden hatches.
I can’t lift the hatch up to take a look because there’s a padlock securing it.  I wonder who has the key?

PS That’s a lecture hall  in the background.