Combining all the records so far into one giant lump:
Yes, the air temperature really did hover around freezing for two weeks in mid-February. If it didn’t require hoisting a four-inch concrete slab with a rusted lift ring, I’d affix a logger to the copper water pipe running across the pit for some direct samples of the actual water temperature.
It hasn’t ever frozen in the last half-century or so; I’m not going to start worrying now…
The Smell of Molten Projects in the Morning 