The Temppeliaukio Church by Timo and Tuomo Suomalainen (1969) is carved out of the rock in central Helsinki. There are lots of thoughtful details to make the whole idea work, including a sort of verge at the edge of the floor, where the wall descends beneath the concrete slab (see second from last photograph, with location specific adaptations to grate).
Hi Nat, it’s a great church, I have been there this summer! If you still there, you should eat here: http://www.anothertravelguide.com/eng/europe/finland/helsinki/destinations/restaurants/atelje_finne/
2 minutes walking distance from the church