I'm very possibly wrong but it looks like a section that was cut out to remove mold. My dad works at a hospital and they have a practice of cutting large swaths out like this if even a speck of mold is detected on the drywall. I believe in the USA their policy for that kind of thing is to cut out 6 inches or about 15 cm above/below the mold growth.
I can't say for sure though, maybe call and ask the previous owner.
I like this answer. I's thinking along similar lines namely perhaps they had a gew drainage issues.
What's a gew drainage issue?
Probably "few".
Have you seen ghostbusters? You know Slimer? The slime from him is the gew.
Jj, a typo, i've no idea what that word is meant to be.
Probably for an air register
Your floor looks like it's a thin layer of concrete atop of a bed of sand O.o
Is this right? What's under the sand?
Dont know. Not sure if I should dig