Keep Plugging Along - Friday 2/3

I replaced the 3 outlets in the den and everything still works correctly!

There is an extra big wire in the box below, however, that

doesn't appear to go to anything.

Any ideas on the mystery wire???

It's the big white one that says OUND 600 on it.

Also, the copper wire wasn't connected so connected it to the ground screw.

Anyway, it all appears to work fine so I hope it's okay.