When I was breaking up an old porch to have a concrete patio poured, I was surprised to find a slab of concrete already there. I started whaling on the slab with my sledgehammer. Suddenly the sledge broke through into a huge, dark void, which turned out to be my basement. Apparently the slab was covering a concealed well room. We now had to replace it. But instead of a simple concrete pour, we had to spend much more on a slab that could span the open space.