For the last 2 days, (since about 0230 6/15), Wetness.pond has been low. Upon inspection, there was nothing obviously wrong (though some of the plastics on the probe are cracked). Reseating connectors and cycling power didn't help. With water on the probe, the signal went from ~0.18V to ~0.0, which of course is the wrong sign.
I've brought the probe back to the base for further evaluation. We do not appear to have a spare here.