Ants in my house

I know this web site is called Ants in my House and not Ants in my Yard, but I thought I would post these pics anyway. I was doing a little cleanup in my rock garden over the weekend and when I turned over this rock I was totally grossed out. I thought at first there was a little bit of mud on the rock, so I wiped it off and then realized it wasn't mud, it was ants! Tons of tiny little ants all over the rock and under it. By the time I got the camera their numbers had diminished.

In my rock garden I have a sand base, covered with some poly (to keep weeds out) and then the rocks on top. I guess this is an ideal ant habitat (living beneath the poly in the sand), it's always moist and cool and the sand makes for easy tunneling.

So I guess my next outdoor project is to rip up the poly? Any ideas?