The annual festival -- where folks make a pilgrimage to the middle of nowhere for music, activities and fun in the sun -- was met with a downpour heading into the weekend ... the last few days to enjoy Burning Man, which now looks to be all but ruined.

Check out these pics -- obtained by TMZ -- showing off the aftermath of a monsoon that came down in Black Rock Desert (where BM is being hosted this year). It's a sloppy mess.

As you can see, there are campers and vehicles parked all around ... but they all seem to be somewhat sunken into the ground on account of the mud that's formed after the rainfall, which had gotten started the night before and trickled into Saturday itself.

The rest of the landscape certainly doesn't look Burning Man-friendly -- and there isn't really a soul in sight either ... normally, everyone would be out and about enjoying themselves.

No word on how long this rain-out sitch will carry on, but people on the ground tell us attendees feel quite trapped right now ... as the rain and mud have made moving around virtually impossible. and folks have been instructed to stay put until the conditions subside.

The worst part - this might be the reality for another day or 2, as it's expected to continue raining -- at least according to some forecasts. TBH, this sounds like an utter nightmare.

Here's hoping the Sun God comes to these free spirits' rescue ... Burning Man's almost over.