Photo: ROBYN BECK / AFP / Getty Images
It's called "super fog" - fog mixed with smoke- and it caused a deadly multi-car wreck in Louisiana killing seven and injuring least 25.
The crash took place on Interstate 55 near New Orleans, involving at least 158 vehicles.
A long stretch of Interstate 55 is expected to be closed for the " foreseeable future" according to police.
The super fog was caused by smoke from marsh fires in the area, with visibility near zero in some places.