Mysterious Fog in the Caves

In the Ninteenth-centruy beer caves (storage rooms for kegs) of the old abandoned German American Brewery located at Main Street at High Street in Buffalo, NY. This photo was taken just days before the facility was torn down and these storage cellars were buried as the ten story building that was once a grain-elevator and malt-house was torn down. The room with 25-foot high vaulted ceilings was clear and we could see from one end to the other.
But suddenly, a mysterious thickening fog began to form in front of us. It would appear for five-minutes and then disappear again, leaving the room clear. At times the fog would get so thick that you could not see the other end of the 100-foot long room, buried more than four-stories underground. At one point the fog was so thick we could not see the way out.
No one had been down in these so called caves in over 70-years, so it was a little bit eerie. The room had been sealed of by a brick wall for decades. The air was so stagnant in there that it was hard to figure out why the fog kept appearing.
QUESTION: What caused the fog to appear?