Like others have said, that mound is basically a vent shaft. Too deep to push the sod up, so they must get the dirt out of their way. When the vent becomes too far from where they're digging, they plug it with dirt and dig a new vent. So then you can use the mounds to connect the dots to determine the direction of travel. Typically the bigger the mound the deeper the run. Often the giant mounds are in dry conditions. You have two options then, dig down to the run, or water to bring the moles back to the surface.

A friend of mine told me the mounds are the result of a male digging a breeding chamber.. but he's also told me some other "tales" that have never proved out.
That said, we've had tones of rain this summer (like 5 inches in one week) but I don't see the tell tale tunnels as I did last year.
Gonna see what I can do with the "connect the dots" method. I caught 8 of the little demons last year and still have 3 or 4 creating havoc around here.

Thanks for the tips!