Old septic tank under wall

[Kent Verge]

While peeling up stick-on tiles stuck to the concrete slab on an addition to my recently purchased house, I uncovered a 2' square hole peeking out from a wall. I peeled away the tile in the next room, and it covered a 2' square of plywood covering an access hole to what looks like an old brick septic system that's probably 5' deep, 5' wide, and 8' long (I didn't climb down in to measure it). The house was built in the 40's; sewer was installed in the neighborhood about 10 years back, and at that time, you had to fill in your old septic tank when you hooked up to sewer. Apparently this was an even older tank.

I plan to fill it in with dirt then seal off the access hole with concrete. Does that sound like a good plan? Has anyone else faced something like this? I bought the house "as is," but does the seller have any liability since he had to know about the defect and didn't disclose it? Stick-on linoleum literally covered part of the hole with nothing underneath.