I didn't realize that either (it sounds unbelievable but it's not). It (may) also explain why radon is such a big deal for some people. There's that much U belo

I didn't realize that either (it sounds unbelievable but it's not). It (may) also explain why radon is such a big deal for some people. There's that much U below you (and much more as you go through the crust), if enough of it is "hot" and releases Radon (which I believe is a decay product, might be wrong) it can build up enough to poison the occupants of the house.