The machine-gun rattle of a jackhammer sends a hail of concrete chips flying in the dim hallway of the Blacker House dormitory at the California Institute of Technology. A bespectacled undergraduate pauses to explain why he is carving through a mass of concrete that blocks a dorm room door.
His answer might as well be a physics problem, for all it means to an outsider. ''There was no microdot on the note, so . . . ,'' he says, holding up the jackhammer as if it were the only logical explanation. Ingenuity seems to be an automatic reflex for the young student and his classmates, who don't consider the incongruity of a cement block barring a door - instead they are challenged to remove it.
Some college students pile into phone booths. But that's penny-ante stuff for these Techers, whose state-of-the-art pranks are showcased every spring during Senior Ditch Day - an annual rite in which all seniors must leave campus for the day or risk being chained to a tree. But they leave behind ''locks'' on their dorm doors they hope will be impenetrable. It is the underclassmen's duty to try to pick the locks.
The tradition produces equal parts of brilliant humor, contemplation verging on scientific breakthrough, and high jinks that spill out of this academic island into the quiet surrounding surburb.
''And to think I almost went to Berkeley (University of California at Berkeley),'' muses one Techer in another dorm not far from the jackhammering. Meanwhile, outside Blacker House, past a tree with a shopping cart lodged securely in its upper branches, past the stone archway with the inscription ''Doest thou love life; then do not squander time,'' a dozen students stand on their heads while others drop a physics book off the top of a multistory building. The book contains several peppermint patties between its pages - and their configuration upon impact will have a bearing on how a lock will be opened.
* There is a thin line between absurdity and genius. And Cal Tech seniors must walk that line every spring before they walk the ramp to receive perhaps the hardest-to-get diploma in America.
Senior Ditch Day is a tradition required of every senior - with the prerequisite of three years of trying to figure out his predecessor's ''stacks.''
A stack is the student's term for a lock. Cal Tech spokesman Dennis Meredith explains that the term comes from the first ditch days years ago, in which material was stacked in doorways and windows to block entrance to a room.
Mr. Meredith says today's locks include brute-force stacks that require more brawn than brain to remove. This year's stacks included one that had to be drilled all the way through, a concrete block with a key inside it, and another that successfully prevented entry through doors or windows, which was broken when underclassmen drilled a hole through the ceiling, finding one spot not protected by thick closet doors.
Finesse stacks, he continues, involve exotic locks that can be rigged to heat , light, sound, water pressure, magnets, or computers. Once a lock was even rigged to a snake. In this case, the snake had to be enticed, through a secret series of training commands, to coil around a box with the key in it. The snake then had to be hoisted from within the room to a spot where the students could get the key.
Others this year included a cross-campus scavenger hunt for a room key by way of a computer programmed with a three-dimensional map of the campus. Another, around a corner from a repeating tape of the periodic table of the elements set to the music of ''The Pirates of Penzance,'' involved a scavenger hunt by way of a lead plumb. The shadow cast by the plumb, which changed periodically, pointed to specific points on a map of the campus where clues could be found.
Finally, the honor stack requires no lock at all. Doors are unlocked but puzzles are left that must be solved by underclassmen, who are on their honor not to cheat. One year, says Meredith, a senior coolly left his room unlocked and a single physics problem on a piece of paper taped to his door. The room was never entered though, because the problem couldn't be solved - not even by Nobel laureate Richard Feynman, a professor of theoretical physics, who was enlisted to help the underclassmen figure it out.
Further, Meredith explains, seniors leave ''bribes'' of food or beverages in their rooms to persuade underclassmen not to create a ''counterstack,'' or some kind of retaliation. While seniors spend months putting together ideas for their stacks - some are probably already designing next year's - underclassmen have only from 8 a.m. to 5 p.m. to crack the locks and come up with some form of retaliation. Because there is so little time to dream up and execute reprisals, counterstacks often take the form of elaborate pranks. In previous years, seniors have returned to find their furniture in the right place but bolted to the ceiling. And once a senior's Porsche was dismantled, then reassembled in his room with the engine left running.
Stacks have changed with technology, but the evolution of Senior Ditch Day stacks has made them a part of scientific community folklore.
The legend of Cal Tech Ditch Day, for example, did more to capture Carlos (Moose) Mussenden's imagination than any college recruiter.
''When I was about 6, I read in Popular Mechanics about these weird guys who booby-trapped their rooms and left,'' the sophomore computer science major from Puerto Rico says of his first enticing memory of Cal Tech.
It's an unusual child who would read Popular Mechanics and an equally unusual young adult who would end up playing intellectual high jinks at this college. The Cal Tech image is one of rigid academia, with 20 Nobel Prizes shared by alumni and faculty here. There is no sports program of note (one student cracks that when he sees another classmate running past, ''the only time you see anyone run around here is on Ditch Day''), the male-female ratio is 6 to 1 on the male side and Pasadena is a 20-minute drive up a smoggy ravine from the glitter of Los Angeles. Even if the image were more enticing, few young people would even consider applying to Cal Tech, which admits students with an average college math entrance exam score in the 99th percentile nationwide (97th in the verbal exams).
On Senior Ditch Day, one of those occasions when Cal Tech's image gets an airing out, a layman can't help but sense the gestation of genius here, even if the humor of most of the activity is too cerebral to understand.
Mr. Mussenden spent this year's Senior Ditch Day wandering the halls of Page House monitoring progress on the solutions to that dormitory's dozen stacks.
''I'm a generalist,'' he explained after several groups of fellow underclassmen enlisted his help on various stacks they were trying to break. He'd have to be a generalist - the variety of problems left behind by seniors represent practically every scientific discipline.He singles out one stack left by Sandra Loh, whose major, physics and literature, is nearly a contradiction of disciplines. ''This stack has nothing to do with physics, it's pure whimsy,'' he says. Her honor stack, he explains, is a puzzle for puzzle's sake that ''glorifies literature'' and has little to do with a cause-and-effect process that will open her door. Part of her puzzle, he notes, ''maligns physics'' by requiring a correlation of the circumference of a student's head to his favorite Greek letter.
Mussenden then points to another stack farther down the hallway where students have reached an impasse. ''Here we have to play a game of bridge (in order to obtain the next clue to solving the problem), but no one knows how to play bridge . . . around here, only seniors do, and they're all gone,'' he explains. In other stacks, there are pure scientific problems that, if solved, will literally unlatch a door. Ruddock House senior Tom Berto left underclassmen to slave over a board with 70 threaded pins mounted on it. Each pin had a clue about whether to turn the pin clockwise or counterclockwise. Turned correctly, the pin sent a marble dropping into a can. The can would ultimately become so heavy that it would tip a jug of liquid nitrogen it was hooked to. The liquid nitrogen would spill onto a heated cross lever that was just long enough to hold the door shut. The liquid nitrogen would cool the cross lever enough so that it would shrink about a quarter-inch, allowing the door to open.
A cinch or not, every puzzle, lock, and barricade warrants the rapt attention of underclassmen who spend the day wandering from problem to problem working on the theory that two or more heads is better than one. Says one computer science major with a diamond stud in his ear, ''We'd never meet these challenges if there wasn't a lot of participation and cooperation.''