The hollow wall

In Council Crest Park in Portland, Oregon, at the top of the knoll there is a large paved circle enclosed by a short stone wall. The area has a couple dedications on it, and there is a massive compass of inlaid marble paved into the area, so you know which way you're looking. Stand on the west compass point and walk over to the wall it is pointing to (this is easy because there's a line in the pavement coming out from the point to the wall). Now look at the stones that the wall is made of. Moving south along the wall, count 10 stones along the bottom row and then give the 11th stone a nice hard kick. As you will easily hear, this stone is hollow. Feel free to kick all the other stones everywhere else in the wall... the 11th stone is the only hollow one. What's hidden inside? As yet, that remains a mystery.