IIRC this was a reddit post, but without the Snapchat text. I do have a vague memory of the OP being from Britain and that they came to the conclusion that it was some sort of sediment tank.
Elevating the most realistic comment from the linked thread:
This is quite common for extensions in the UK at least. If there is a man hole you can often get permission to build over it but you would need to be able to provide access should it be needed. In the 15 years of living in a houses with one under our kitchen no one has ever needed access. These is one just outside in the side passage so likely most blockages should they arise could be cleared from that one. A previous property was similar but they had tiled and concreted over so would make a mess should they ever need access.
Edit: one in our kitchen, if access was needed they would need to move kitchen units and break grout. https://imgur.com/gallery/OU0jFvk
IIRC this was a reddit post, but without the Snapchat text. I do have a vague memory of the OP being from Britain and that they came to the conclusion that it was some sort of sediment tank.
Oldest instance of that image from Tineye.com
edit: https://www.reddit.com/r/WTF/comments/gj1gea/i_bet_this_wasnt_on_the_zillow_page/
Elevating the most realistic comment from the linked thread: