I've been using a radon detector (National Safety Products Pro 3) to
check radon levels in our house. Levels are high, but within my
expectations for our situation (25 in basement, 15 upstairs).

But, then I put a hole in the cellar floor, dug out a 1' deep hole, and
put the detector down there. It's getting readings of 300+. I'm kind
of surprised at this If anyone has any insight on 'in ground' radon
measurements, I'd appreciate hearing.

(NB: It would be OK to spare me the 'OMG, evacuate the house and call a
HazMat team" comments. Bandwidth is a precious resource.)

TIA,
George