I guess when they got the cleaning the City, they cleaned up that alleyway, too, because when you pass by it, the stench no longer causes you to want to vomit, but having been there in the early-to-mid-90s, I still won't venture into it.
It's situated on Ste-Catherine West, right behind a Burger King that been there for ages and what is now the downtown core's biggest Future Shop, although we used to know it more as the block that housed Labyrinth (rock t-shirts and posters galore, knives and lighters and motorcycle stuff too) and Mars (a dusty asthma-inducing basement - even though it was on the second floor - where they would sell bootleg CDs for almost $100 and one-off collector's show posters from the 60s to the 90s) - still, it's an area that most Montrealers passed by on a daily basis.
You could smell it from almost a block away, putrid, like decomposing homeless bodies that had shat themselves prior to dying there. When you'd see a hobo walk in there, you just knew he'd never come out - you had probably witnessed someone's final moments in the World Of The Living. Whether it was summer or winter, there was an acidic steam coming out of it that would burn the eyes, a somewhat transparent cloudiness of stench that stood from the ground to about twelve feet high - and it stank so much that most people who would walk past it would just hold their breath to not be subjected to it in the pit of their lungs and settle for teary eyes and temporary burns to the face and hands; it's a surprise a fast-food joint could survive and thrive there, considering.
A few friends and I entered it once, as a dare, the one time that it didn't smell like the pits of Hell had thrown up a sack of shit and sweat - we must have been 14 years old, three or four of us. It was a Sunday. We saw it to the end, it lead to a parking lot where the cars that were closest to the alley were rotting, like its extension into the world of mechanics, like they had caught a mutated version of whatever virus lurks in the air of the alley.
We didn't want to stay too long to not catch it too, so we quickly fucked off to Labyrinth, but we were uneasy being there despite it only smelling like decayed garbage and stale urine that time. You should have seen the look on people's faces when they saw relatively healthy teenage boys come out of there: shock and - not awe, maybe mistrust. They even stayed clear of our way until we entered the store, it was weird and pleasant at once, like we were Ghost Riders or something.