In my concrete basement floor, there is a rather large bolt and washer. The washer is about 4 inches in diameter, and the bolt has a 1 inch head. What is it?
All I can think of is that it's some device to keep this from happening:
That's likely a cleanout for your sewer line. When opened, you can run an snake down the line to remove any blockage without ripping out your entire foundation. Here's what it would look like from the side:
And the cap itself can have different styles: