What was that place?

I had no idea what happened or how the main character all of a sudden teleported onto a small swamp island that exists somewhere just beyond the basement, but before the exit into what normally would've been outside, if not for all the water. It made absolutely no sense at all. Can somebody explain what that place was (I'm talking about where the eggs and bodies were). If it was still part of the house, why would it have a completely dirt covered, lumpy area like that, that looks like outside, other than the fact that it was covered.