Is it a vug under the rug?
Most likely the pad shifted and bunched up under the carpet. I’ve even seen old pads that had deteriorated and were crumbling apart.
The floor is supposed to be swept clean before putting down the pad and laying the carpet. Any dust or dirt can come up through the pad if the floor was dirty.
A firm lump the size of an 18 inch dinner plate? That’s a lot of dirt!
It’s probably a clump of foam pad that’s deteriorated, come apart, and wadded up.
Did you have all the plates you needed?
And it was…water swollen sub flooring.
I’m actually kind of disappointed.
The AC unit must have leaked because the subfloor was damp, but the rug still seems to be dry, so I donno.
We cut into the rug (thank you for the idea deltasigma) pulled it back and the subfloor was all lumped up. I got the drill & and a spade bit, drilled through the lump - into nothing. So now we really will have to pull up the rug, cut out the lump and put down a new piece of subfloor. Not the holiday weekend I had planned. 
Whew! At least you will now be able to sleep at night without wondering what is growing…under your rug…just inches from your bed…
Oh well. My money was on it being a strange grid ball.
This is true! The grown up part of my brain knew it was something mundane, but the 5 year old part made me not step on the lump when I went to bed.
How do we know this is really Silence Lenore posting? Maybe someone else–or something else–is posting using her account.
Maybe these last few posts–all nice and reassuring–were really posted by…
THE LUMP.
You never know! It could have been like that Stephen King short story, “The Raft,” where the people in the boat all got eaten by what looked like an innocuous little oil slick in the lake. Better safe than sorry.
I strongly suggest finding the source of the leak first. Good luck.
.
“Harold! So that’s where you’ve been! And there’s my old hairbrush, too!”
I’d suggest replacing the strand board with real plywood. Strand board is notorious for coming apart if it gets wet. I had a roof leak in a bedroom and had a bad area in the floor. Just like the OP described the wet subfloor was just mush. I made the repair with plywood. If it gets wet again then fans can dry the plywood without harm.
Or “The Mist”, which is the exact same story. Just replace the oil slick with fog and the boat with a grocery store.
Coming soon : John Carpenter’s ‘The Lump’, starring (really old) Kurt Russell and the guy who’s always in these kinds of things and that other guy, too.
“First goddamn leak of summer.”
“We’re gonna need a bigger towel!”
I am sick of these motherfucking lumps on this motherfucking floor!!