"So! ...So - you think it amusing to turn a school corridor into a swamp, do you?"
"Pretty amusing, yeah..."
-- Dolores Umbridge and Fred Weasley, just before the Twins flew away into the sunset (OP29)
"If anyone fancies buying a Portable Swamp, as demonstrated upstairs, come to number ninety-three, Diagon Alley - Weasleys' Wizarding Wheezes ...Our new
-- Fred Weasley (OP29)
When deployed, creates a swamp covering the target area. This was the Wheeze that resulted in Umbridge’s famous failed detention.
- The portable swamp eventually filled a complete corridor on the fifth floor in the east wing at Hogwarts, and the Twins left no instructions for removing it. The teachers didn't seem to mind that it made Umbridge furious (OP30).
- Filch ferried students across the swamp in a boat.
- Professor Flitwick eventually removed it, but left a small bit roped off because "it was a really good bit of magic" (OP38).
It's possible that the Weasley Twins used some of the Stinksap from Neville's Mimbulus mimbletonia to create their Portable Swamp. When Umbridge confronted them about it in the corridor, Harry noticed that "Students were standing all around the walls in a great ring (some of them, Harry noticed, covered in a substance that looked very like Stinksap)... (OP29)