Working in construction on ladders and above drop ceilings, I see many ducts. Here's the thing. Some vents may be large enough to crawl through, but for the most part, they lead to grates bolted to the wall, ''fans'', heat exchangers blocking the way, or to vents in the drop ceiling. And they're supported by thin metal straps ''nailed'' to the concrete ceiling (using a special nail gun). They'd pull out of the ceiling if you tried to put your weight on them. If you tried to rest on a vent in a drop ceiling it would buckle under you, since drop ceilings are held up with wire and thin aluminum strips. One other thing. To assemble metal ducts, the mechanical/HVAC contractor uses sheet metal screws. That point inwards to the duct. So at every joint you'd hit a row of pointy screws pointing in from all sides.

While the area above a dropped ceiling can be cramped, the area below a raised floor is often more so. Due to the floor needing to be weight bearing, and not disrupt the flow of rolling chairs, the tiles are also often far more sturdy, and harder to remove. You can also support your weight on the concrete underneath. This combines to make crawling under tiles to string cable often far more convenient, if also far less comfortable, than simply removing all of the intervening tiles.

This troper's dorm had normal sized air ducts, but the air return was through louvered doors leading to the machinery. And it was not only big enough to go through (watch your head) but had a chair and light that someone had rigged, right in the middle. Rather ''RealGenius'', and ''the'' place to relax after a Texas summer's run-to-classes.

This troper attends MIT, where there's an active "hacking" culture (read: going places students aren't supposed to go). Air ducts and similar structures are often surprisingly easy to get into.

So thats were Gordon Freeman went for MIT

This Tropers father worked as an air duct cleaner when he was This Troper's age, and he says you had to crawl through them to clean the properly, and this was in the mid-to late 80's.

This troper's boarding house at school had floor voids that could be accessed fairly easily from a storeroom in the basement and which ran beneath most of the ground floor. In the parts that weren't carpeted or otherwise covered you could push things up between the floorboards ...

In a similar vein, this troper's high school Latin classroom was a converted conference room/storage closet in the library. Surrounded by nonstructural walls and with a drop ceiling several feet lower than that of the library outside, it was perfectly designed for a bunch of bored high schoolers looking to climb up into the ceiling and explore. One classmate used the rafters as monkey bars to crawl into other parts of the school, and I spent a memorable three minutes hiding by the hole and trying not to breathe after the librarian walked in while I was upstairs. The best part was when we brought back a six-foot long piece of scrap lumber and wrote "HALL PASS" on it.

Similar to the above: When working fast food, there was an argument as to what station the radio should be on for our closing shift. After several people's attempts to sneakily change the station, the manager put it on her favourite, pocketed the remote, and locked the door to the room with the controls. This troper climbed into the drop ceiling, went over the wall into that room, covered the remote sensor with a piece of paper, switched it to his preferred station, and then got back out clean. The kicker? The manager had locked the keys in the office, and a repeat had to be staged - once a promise was made not to change the station.

This troper, on Halloween at his University, observed two men dressed as Teenage Mutant Ninja Turtles crawl out of a sewer grate in one of the campus's most populated areas. It was rather... surreal.

The International Spy Museum in Washington DC has an oversized fake airduct to simulate this. This 5'7" 160 Lb male troper could crawl comfortably on his hands and knees inside, but there's a panel that gives way under your weight if you do that, causing a loud THUNK. This troper found he could prevent the noise by instead crawling arm-over-arm along his belly.

One particularly ingenious fourth-grader managed to pull this one off in this troper's grade school. Subverted when he managed to not only fall out of the vents, but fall right through the ceiling of the ''principal's office''.

This troper, at the age of twelve, could frequently cut Sunday school by fitting her entire (130 pound, 5'3" at this time) body underneath a heavy '''concertina partition'''. And then she hit a bump.

This troper attended a middle school that had this thing where there was a hallway with these big heavy (and opaque) doors across it, which were always open, with the edge of the door against the wall, during school hours; however, there was a bit of wall between the hinge of the door and the wall 90 degrees to said doors. Both of them therefore had little triangles of space between door and corner that were perfect for a skinny twelve-year-old girl who loathed gym class and had absolutely no trace of claustrophobia to disappear into, since they weren't too hard to move and they stayed 'shut' (I think they used magnets or something).

At this troper's college there are (last he heard) at least two sets of keys to the maintenance tunnels floating around somewhere on campus. This is after at least one set got turned in by an alumnus. And of course there are a few doors into said tunnels which never seem to be locked.

This troper (and some friends) once stole a teacher's pointer stick. We then ventured to the far side of the building during a break, crawled into an air duct to a small crawl-space ''above'' the classroom. Very silently, we removed one of the tiles in the corner. When class started again, the teacher was, of course, looking for his pointer stick. So I, being a cheeky bastard, decided to poke him in the back of the head with it. ''Several times. Never once getting caught.'' Eventually, the class just burst into laughter - including some of my buddies in the crawlspace. The teacher was ''livid''. I then decided to bop him on the nose with the pointer and tell him to " Be quiet, fool, ain't you know it's top-secret guvvahment bizniz in 'ere?!" (With a fantastically faked American accent.) Cue the teacher joining in the laughter. We did get suspended, though. Worth it.

This troper's college definitely does not have steam tunnels but if they did, they would be surprisingly navigable, though some would get rather tight, and some would just lead nowhere. There might even be one that goes past the bottom of the university swimming pool... right by a window enabling adventurers to see the bottom of the pool. Though if there actually were steam tunnels, this troper would be the last to know...

This troper had a teacher who used to teach mentally retarded kids. One time, when the teacher's back was turned one of the boys managed to climb the wall ''without the aid of any objects'', get to the rafters, crawl through the rafters for awhile to the top of the computer classroom. The kid fell, falling atop of the computers, in the middle of class. After a beat, the computer teacher said, "So glad you could join us, Jake."

This troper's High School had at least 2 instances of accessible spaces of this nature. One was a place with a lowered ceiling of such height that you could STAND UPRIGHT inside of it. The only safe place to walk was on top of one of the walls underneath the fake ceiling, but it was still cool nonetheless. The second was a maintenance crawlspace above and behind the building's furnace, where many a young couple would spend some private time together.

At this troper's university, exploring the maintenance/steam tunnels is considered an unofficial graduation requirement. While getting in can involve rather tight squeezes, the tunnels themselves are fairly roomy.

This troper's university has an old gym which holds all sorts of wonderful crawlspaces and hideaways. By climbing out a window you can get on the roof, where there are several trapdoors that lead into an insulated area over the main gym where the ceiling reaches around 30 feet at its highest. From there you can get into several offices. There's also an abandoned pool, which the janitors seem to think they can keep us out of for some reason. Heh.

This Troper has never done anything like this before herself, but... Her school has panels in the ceiling that can be removed, revealing apparently a lot of room. She's never been up there herself, though. Anyway, the ceiling is too fragile to walk on, but you can put stuff up there. One time some kids stole their math teacher's cell phone during lunch and put it on the panel above the teacher's desk. Everyone then proceeded to call it the entire hour during class, causing it to ring/vibrate with said teacher having no idea where it was as it was right above him. They eventually told him where it was, but didn't even get in trouble for it. Best. Math. Class. Ever.

Happened to someone I knew last year in theater - the girls' and boys' dressing rooms are adjacent, connected only via those paneled ceilings. Well, one day one of the guys decided to see if the steel framework would hold his weight. He not only succeeded in getting into the ceiling structure, he crawled into the girls' dressing room. This led to a troupe-wide reprimand from our director when she found out.

I work in a facility that is rather GenreSavvy about this and has motion detectors to detect people trying to sneak in this way. Said motion detectors are on 24/7 and never turned off even during maintenance that requires people being up there. Needless to say, using this trope at my job is not going to work.

I was always too self righteous to do this as a kid, i never tried it but there was actually a lot of little nooks and cranny's that you could use to hide in at my old school. now that i'm possibly going to college this is becoming one of my plans, that and also a good game of assassins vs. humans vs. zombies.

This troper once had a music lesson with no teacher. The ceiling in the music room had a hatch to get into the attic space of the building. One lad in said class decided it would be fun to climb into said attic space.