Severus Snape turned around a corridor that was more dusty than it ought to be. Bits of cobwebs clung to doorframes and corners. It was far more complicated than a Notice-me-not spell (which was layered atop the other spells), but also far more effective. There were plenty of disused corridors kept sparkling and squeaky clean at Hogwarts after all. You had to deliberately 'fix' the house elves in order to create the proper atmosphere for 'Do Not Enter.'
The fifth door from the end of the hall on the left. Snape came to it, and discretely checked that there were no observers, out of the corners of his eyes. No movement, at least.
Snape entered the room, which was coated in a thick covering of dust on nearly all surfaces. On the floor, inside a circle of throne-like chairs, was a Persian rug. Or at least so it appeared. Snape had acquired it straight from Iran. He knelt, not on the rug but in front of it, and rolled it up. In the process, he disabled three traps he'd set there himself.
Beneath the rug was a trapdoor. Snape looked it over for signs of tampering.
Nothing had been disturbed.
Smoothly, Snape slid on a pair of dragonhide gloves, before opening the trapdoor.
From below, the dank scent of the mire drifted upward. Incongruous for a stone castle, the humidity was a cold and clammy balm in the chilly dungeon.
Snape descended down a metal ladder, every rung creaking or whining under his weight.
With a gesture from his wand, the wooden door above closed, then the metal trapdoor, and then the rug rolled on top of it.
Softly, growing louder, Snape sang:
The heat is on, on the street
Inside your head, on every beat
And the beat's so loud, deep inside
The pressure's high, just to stay alive
With every beat, his heels splashed in the swamplike corridor, and the corridor itself grew warmer.
"Bufo," Snape crooned, "Where are you buddy?"
There was a brief spurt of motion, behind a swamp Tupelo.
"There you are," Snape said, striding around the morass of roots. "It's time to wake up now."
[a/n: An experiment in a more Lovecraftian style. Do you like it?
Also, ideas on what Snape's up to are, as always, welcome!]