An old vinyl floor had been glued to the subfloor. Under the tile. Under the board under the tile. And it was trapping all moisture. The subfloor is GROSS. The house smells SO bad that I kept Ben outside on the back porch all morning and then asked Julia to come and take the kids to the park all afternoon. Then the kids and I are going to a hotel for the night.
*Update* There were TWO layers of subfloor. Yup. That's right. Support Beams, Subfloor 1, Subfloor 2, Vinyl Floor, Underlayment, Tile. Gotta love old houses.