A thick, 'mysterious' fog has descended upon a wide swath of the US, sparking fears of another 'Operation Sea Spray.'

For one week in September of 1950, the US Navy sprayed massive amounts of bacteria into the air two miles off the coast of San Francisco, California.

The aim of this secret biological warfare experiment was to learn how vulnerable large US cities like San Francisco would be to a biowarfare attack by terrorists.

The bacteria used were Serratia marcescens, which can cause respiratory issues and meningitis, and Bacillus atrophaeus that can be lethal immunocompromised individuals.  At the time, the Navy believed these bacteria were harmless to humans.

But when Bay Area residents began rushing to the hospital, it became clear that was not the case. 

After inhaling thousands of bacterial spores, 11 people checked into Stanford Hospital near San Francisco with very rare, serious urinary tract infections that doctors ultimately determined were caused by the experiment.  

One patient, a man named Edward Nevin who was recovering from prostate surgery at the time, died. 

Now, 75 years later, some Americans fear this may be happening again as they claim an 'unnatural' fog with a 'burning chemical-like smell' has plagued their communities in recent weeks.

Concerned citizens have flooded social media with videos of what looks like white particles raining down and swirling in the air. 

Some witnesses have also claimed the fog has triggered health symptoms similar to a respiratory illness.

A Florida resident told DailyMail.com that they stopped at a gas station for about 10 minutes and began feeling ill.

'Within about and hour, I kept sneezing over and over for about three hours, and my eyes were really puffy,' she said.

'I got very warm and I felt like I had a fever, and my stomach was cramping.' 

A woman in Kansas also shared that there were 'massive amounts of chemtrails' over the area in a week, followed by a dense fog.

And a California resident shared on X Monday: 'Here in SoCal, unusual fog the last few days. Strange smells, but I've been noticing the smell for months now, burns my nose. Seems like an all-out assault from the skies the last few days.' 

Parts of Texas, Wisconsin, Iowa, Maryland, Virginia, West Virginia, Nebraska, Kansas, Oklahoma, North Dakota, Florida, and Minnesota were under fog alerts when the bizarre reports surfaced this week.