A lot of odd things happen on the subway—brie cheese feasts, for instance, or zoological experiments, or seamless service (the oddest phenomenon of them all). Yesterday, one Redditor's roommate witnessed his share of strange when the L train he was on filled with a mysterious dust, or is it smoke?

The poster told us his roommate, who filmed the above video, noticed the clouds at the Jefferson stop in Bushwick:

Basically the train stopped at the Jefferson stop on the L, the doors opened. They were there for 3-4 minutes. Some sort of dust, or smoke poured in from the ceiling of the stop and filled the train before they closed the doors and got moving. He thinks it most likely came from the vents in the ceiling from the street. He did say people in the train were very alarmed. One women was screaming for the train to start moving and others ran off as soon as it started happening.

Another Redditor wrote that they noticed smoke, too, after getting on an L train at Morgan Avenue. "It didn't have a smell. It was more like a chalk dust mist than it was smoke," they posted.

At first, it seemed possible the smoke was part of the Department of Homeland Security's biological terrorist attack airflow tests, which were announced earlier this month. But when reached for contact, a spokesperson with the DHS told us the tests were completed last week and the gases used were odorless and colorless, so the mystery is still afoot.

The MTA did not respond to request for comment.