Minus thirty-three.

Well, as previously noted, Joe Taylor says of WSPR:

"The protocol is effective at signal-to-noise ratios as low as –31 dB in a 2500 Hz bandwidth."

The new WSJT-X version is meant to be of 'increased sensitivity', but we aren't told how much more sensitive.

I said earlier this month that I'd seen infrequent but regular decodes as low as -33dB in the past, using older editions.  I saw one again today at 18:10UT with the new edition, but they don't seem to be any more common than in the past.  The next spot of WB2CPU (18:14UT) was at -28dB, so the -33dB event doesn't seem to have been down to strong adjacent signals.