This is how a group of whiz kids, using a mainframe computer as big as an elevator, messengers stationed at pay phones and a pair of subway tokens, staged a wild race against time under the streets of the Big Apple.

Massachusetts Institute of Technology student Peter Samson needed something to spice up his spring break in 1966. Life inside the rent-controlled apartment he shared with three of his MIT classmates on Manhattan’s Lower East Side had been a little boring—until inspiration arrived on the back of a New York City subway map. Samson scanned a brief blurb about a youngster from Queens who had spent more than 25 hours straight riding every subway line on a single token and knew he had found his next adventure.

While fellow spring breakers frolicked in the sand and surf, Samson and his colleagues from MIT’s Artificial Intelligence Laboratory devised a plan to ride the entire NYC subway system in record time. “I liked trains; I liked computers; I was then exploring New York and wanted a reason to see the entire subway system,” Samson says of his reasons for undertaking the challenge.

To plan their ride, the strap-hanging MIT students rode the rails and made multiple visits to the New York City Transit Authority in order to gather schedules and confirm train times. Complicating the planning, certain subway lines ran only at certain hours. Using a computer, Samson’s team choreographed the most efficient route, down to the precise second.

At 6:30 a.m. on March 31, 1966, Samson and fellow students George Mitchell, Andy Jennings, Jeff Dwork, Dave Anderson and Dick Gruen deposited their 15-cent tokens at Brooklyn’s Pacific Street Station and began their journey. For more than 24 hours straight they rode 77 different trains through every subway station and over more than 400 miles of track, equivalent to a trip from New York to Pittsburgh. Although there were some bumps along the way—Samson, who was in possession of the route, was left behind at one station—the plan was never seriously derailed.

“Probably some people looked at us racing across the platforms and said, ‘There goes a bunch of teenagers up to no good,’” Gruen told the Associated Press. “But we love subways and to us this was an adventure.”

When the six weary riders stepped onto the platform at their final stop, Pelham Bay Park, on the morning of April Fool’s Day, reporters told them they had missed the record set in 1964 by New Jersey teenager Geoffrey Arnold by a single minute. No joke.

Having completed the trip a half-hour slower than he had hoped, Samson immediately set out to launch another attempt at the record. One of the first things he did was to form the “Amateur New York Subway Riding Committee” to codify the rules for what constituted an around-the-system ride. With Arnold’s assistance, Samson and Gruen developed detailed regulations that were recognized by the Transit Authority.

Samson recognized that one of the failings of the first attempt was the inability to employ computing to update the route in real time as unforeseen circumstances occurred. For the next try, Samson harnessed the power of an enormous mainframe computer on the MIT campus to calculate the most efficient route for traversing the subway system.

The job of inputting schedules into the mainframe computer—which was about the size of an elevator—and testing various software algorithms took the better part of a year. “I hoped that detailed knowledge of the schedules and real-time updates would give a faster time, even with the added routes that had to be covered under the defined rules,” Samson said.

While Samson, by then a research assistant at the Artificial Intelligence Laboratory, could rely on the cutting-edge computer technology of 1967 to continually update the most efficient route, communicating those changes to the designated riders was no easy feat in an era before cell phones, let alone smart phones.

Samson set up his base of operations in the MIT Alumni Center in New York with a Teletype link to the mainframe computer on the MIT campus in Cambridge, Massachusetts. Among the 15-person team were runners stationed at specified pay phones and strategic subway stations so that they could report on the riders’ progress and relay updated route information from the control center. “The biggest issue was recruiting and managing more than a dozen volunteers to staff the telephones at the Alumni Center, to run around the system carrying messages to and from the actual contestants and at MIT to keep the computer running,” Samson says.

At 2:43 p.m. on April 19, 1967, Mitchell and Jennings boarded a train at the 168th Street Station in Queens for their second try at the record. Only three hours into the ride, chaos ensued. The computer had technical difficulties, one of the runners missed a stop and the riders were running late after missing their targeted M train.

Samson realized that he faced a dilemma when communicating these changes to the mainframe computer because the first updated route it would calculate wasn’t always the most efficient, which left him having to make a judgment call. “The primary obstacle was that it would take too long to compute the absolute best solution, and we had to balance waiting for a better result against getting a changed route to the contestants when they needed it.”

At the recommendation of the food science department at MIT, Mitchell and Jennings ate a low-residue diet including sandwiches and candy bars, and there are no mentions of bathroom breaks in the log. When the riders completed their circuit of the subway system at the Pelham Bay Park station just after 4:30 p.m. on April 20, they found that their time of 25 hours, 50 minutes and 30 seconds had beaten Arnold’s record by six minutes.

Samson went on to become a computer software pioneer and one of the inventors of Spacewar!, considered by some to be the world’s first video game. Guinness World Records now oversees the record for circumnavigating the New York City subway system, which was held by Matthew Ahn at 21 hours, 28 minutes and 14 seconds until the opening of the system’s new Second Avenue subway on January 1, 2017.