So this has happened to me a couple of times in the last few days whereas I had never seen it before.
Well no, there was a bunker I can’t remember which where one of those rooms with computers and blue tanks (possibly generators but I think some kind of tank) has a metal floor you can fall through, but I never fell through it.
I was riding my bike down the road from the Stenhaga Farm heading towards the bridge just after the scene in the road with a car on a tow truck bed, a dead soldier and a bunch of destroyed runners and cars. Riding square down the middle of the road on the bike when suddenly it fell through the terrain and made that big sploosh sound when it finally hit the boundless sea that is underneath all the land map. It was this that made me realize that the occasional giant toilet flushing/sploosh splash sound was probably machines falling through the terrain nearby.
Then yesterday I spawned to Hinsevik Ferry Pier. I left the safehouse building and walked by the burning cars and suddenly was falling… once again into the boundless sea that lies underneath all the terrain map. The falling would go on and on and then, as when you walk into to deep water, it would attempt to transport me back to the land. Yet it was transporting me back to the same spot of land (a paved area, mind you, with buildings on it) where I would instantly be falling through again. This went on and on and on, never dying. I found I could access the map but I had to be quick to try to fast travel somewhere else to hopefully get out of it. I finally was able to be nimble enough to do so and was about to make it to Sillavik church safehouse, but much to my chagrin the looping continued there. The only way I was able to get it to stop was to fast travel to Yttervik between falls. I thought since Yttervik is so low to sea level that perhaps if I could fast travel at the top of the fall then I’d have enough room to hit land. It worked and I made it through all that without dying.