Our live indoor radiation monitor was indicating unusually elevated and climbing radiation levels inside of our house, a search for the source turned up radioactive rain water leaking into our basement. ARRRGH!
We had this crack fixed a few years back and it had not leaked, supposedly it has a lifetime warranty against leaks, we'll see. In the meantime, this is why you don't want radioactive rain water leaking into your house.
The indoor Geiger counter is located in the middle of the house, on the first floor, above an HVAC air intake. The HVAC system has been off all day. We have kept our windows closed because of the radioactive rainfall, but it does no good when the radioactive rain flows into your basement.