A pipe beneath Mt. Auburn Street leaked large bubbles of natural gas and forced an hour-and-a half evacuation of Adams House last night.

Just before 9 p.m. last night, a passerby on Bow St. reported to the Cambridge Fire Department (CFD) a strong smell of gas. Firefighters arrived a few minutes later and, using gas probes, began to take readings in and around Sorrento Square.

Moments later, a firefighter found what he called an "explosive" reading of gas in the basement of Randolph House, across Plympton Street from the main Adams House foyer.

Assisted by Harvard facilities personnel, the Harvard University Police Department (HUPD) started to evacuate the whole of Adams House "as a precaution," according to Edward Mahoney, a CFD deputy chief.

Andor I. Meszaros '02, who lives in Randolph, said he could smell the gas for at least half an hour before HUPD arrived to evacuate him.

"The officer said there was a gas leak, and that we should crack our windows and get out," Meszaros said.

Officials decided not to activate the building's fire alarm because an errant spot of electricity from an alarm box could have sparked an explosion, they said.

Though most students left immediately, HUPD officials said they took extra precautions to ensure that everyone got out safely.

"Some didn't answer their doors, so we're going door-by-door with a key," said Kevin P. Bryant, an HUPD sergeant on the scene.

With book bags hanging from their shoulders, students walked to Quincy House dining hall, which had been opened for evacuees.

Some of them wore only t-shirts and shorts in the chilly rain.

David Fithian, the senior tutor of Adams, arrived at Quincy House with his dog Buster in tow.

"But we left our cats. That's the big problem," said Michael Rodriguez, Fithian's partner.

HUPD spokesperson Peggy A. McNamara said she estimated that between 350 and 400 students and staff were asked to leave their rooms.

The CFD, which had one engine, one ladder truck and its two rescue companies on the scene, began to take gas readings from buildings along Plympton Street.

CFD officials said they discovered that the gas leak originated in a pipe which vented through Sorrento Square and up Plympton Street.

"We stuffed some rags in the ends of the pipe to stop up the leak," Mahoney said.

Gas company trucks arrived around 9:30 and began to excavate the leak.

In Quincy's dining hall, Adams House students munched on pizza from Tommy's and prepared to watch "E.R." on a television set brought there by a student. Board members of the Bisexual, Gay, Lesbian, Transgender and Supporters' Alliance (BGLTSA), who had been evacuated from their meeting in Adams House, continued their business in Quincy.

By 10:15, gas readings were low enough that students were allowed back in their rooms.

-- Parker R. Conrad and Robert K. Silverman contributed to the reporting of this article.