I have an Inground spa, that has never leaked a drop of water. It is about 8 years old.

All of a sudden last week, the water level drops about a foot in a few hours. I fill the spa up to the top, then it drops about a foot again in a few hours.

The pump in not running at all. I fill it up, and withing a few hours of the water sitting in the spa, it drops about a foot, then stops.

I don't see any obvious cracks, leaks, or water. However, since it is inground, it is hard to determine what is going on outside the spa.

I am thinking that since it drops a foot so rapidly (about 3 hours), then stops and does not drop anymore, that might indicate the location of the leak. It is not the pump or filter leaking, because that is not even running.

I was thinking maybe the PVC pipes running between the spa and the pump are broken underground. The problem is that the back yard is paved with concreate and tile. So to try and dig it up to inspect the pipes would require breaking up the concreate.

Anybody know what would cause this? Or how to troubleshoot this?

I hate to say it but you have all the information to answer your own questions.... the level at wich it stops leaking is the height of the leak. The pavement will need to be broken up if necessary to access the plumbing around the spa. The leak could be a pvc line, but I would almost bet it is at a jet or a fitting.