MEXICO CITY -- In a clean, hushed room in the south of Mexico City, cameras, computer screens and scrawling needles track the symptoms of a special patient, as they have every second of every day for the past two decades. The monitors indicate that "Don Goyo" is breathing normally, even as he spews hot rock, steam and ash.

That kind of activity isn't unusual for the 4,500-metre volcano, Mexico's second-highest, whose formal name is Popocatepetl, or "Smoking Mountain" in the Aztec language Nahuatl. But this volcano, personified first as a warrior in Aztec legend and now as an old man grumbling with discontent, is in the middle of two metro areas, where his every spurt can put 20 million people on edge.

Mexico's National Disaster Prevention Center laboratory keeps a round-the-clock watch on Popocatepetl, with anywhere from six to 15 technicians analyzing data for signs of a full-scale eruption, which they can never fully anticipate.

Though lava or glowing rock would only travel so far, an explosion could be deadly for 11,000 people in three farming villages within 16 kilometres of the base because of landslides and hot gas. A spectacular plume of ash could also wreak havoc on one of the world's largest metro areas, much as it did in 2003, when the sky over Mexico City more than 65 kilometres away nearly went dark in the middle of the afternoon. The neighbouring city of Puebla on the other side of the volcano from the capital would also be clouded over.

"The volcano is like a patient, and we observe the different aspects," said the centre's technical director Gilberto Castelan. "Here we receive over 60 indicators in real time."

The 20-by-30-foot (6-by-9-meter) laboratory resembles those that once housed old giant supercomputers, everything plain white with a server at one end and screens all around. Five remote-controlled cameras positioned on the side of the mountain emit real-time images, while sensors feed data to the constantly scrolling seismographs as the crew and volcanologists analyze the concentration of gases and changes in the shape of the mountain. The loudest laboratory sound is a regular ping that alerts technicians to every seismic shift, at least a half dozen an hour.

The data helps set the "volcano stoplight," a three-colour system in which green means little activity, yellow means warning and red starts the evacuation process -- something that has occurred only twice since 1994, when the volcano awoke again after sitting dormant for seven decades.

"It's one of the most advanced laboratories of its kind in the world, and the scientists in charge are using the best methods," said Michael Sheridan, a volcanologist at the University of Buffalo in New York who has studied Popocatepetl. "It is very difficult to predict the behaviour of a volcano that has not had an eruption in recent history."

Earlier this month, Popocatepetl released ash that grounded plane flights and dusted cars, but it quieted down enough last week for the warning to drop from yellow-3 to yellow-2. The Mexican government has designated evacuation routes and shelter locations in the case of a bigger explosion.

Popocatepetl, nicknamed Popo or Don Goyo, is a stratovolcano, a steep conical formation built from layers of thick, slow-moving lava and ash -- the same type as Mount St. Helens in Washington state, scene of a 1980 eruption that was the most deadly in the U.S., killing 57 people.

Mexico's disaster prevention centre says Popo has been active for at least 500,000 years and has had at least three eruptions as large as Mount St. Helens, the most recent 23,000 years ago. Unlike Hawaiian volcanos and their rivers of lava, the biggest dangers for those nearby are mudslides and swift-moving clouds of gas. For those farther away, it's the ash, which can ruin motors, stall airplanes, cover roofs with material heavy enough to make buildings collapse and cause respiratory diseases.

"Considering the number of people who would be affected, it could be considered among the most dangerous volcanos in the world," said Ramon Espinasa, director of geological hazards for the disaster prevention centre.

According to Mexican legend, Popocatepetl was a warrior who sought the hand of Iztaccihuatl, a fair maiden whose reluctant father told her that her suitor had died in battle. The "Romeo and Juliet"-style tale ends with the lovers turning into twin mountains east of Mexico City. The dormant peak of Iztaccihuatl has since become part of a national park, while access to Popocatepetl is closed off.

Don Goyo, meanwhile, is the nickname for Gregory, a character who supposedly was the spirit of the volcano and would come to warn the locals of eruptions or to assure them that the mountain, despite plumes of smoke, was calm.

Today that's Castelan's job. He and his crew of technicians don't have much to say about the myths or legends, preferring to stick to the hard data in their laboratory, which opened right after Popo's reawakening two decades ago. At the time, Mexico was about to plunge into one of its worst economic crises. Since then, Mexicans say the eruptions are just Don Goyo showing his discontent with the course of his country, including blowing off smoke and ash a year ago, just before the presidential election.

Castelan prefers to look to the sensors to read Don Goyo's thoughts. The trick is monitoring the crater, where it's too hot for instruments, and that's where the seismographs offer clues.

Some tremors indicate an internal buildup of magma, while others result from expulsions of rock and ash. At times the only way to really see what's going on inside is to fly over the crater, something Mexican officials do regularly, feeding the laboratory more data.

The technicians are especially watchful of lava domes that can form inside the crater in hours, days or weeks, creating a pressurized cap.

The domes usually grow and then collapse. But they could also harden into a sort of bottle-stopper, allowing pressure to build until the volcano violently dislodges the cap in an explosion. What seems to be happening with Popo is lava settling inside, bringing the crater floor closer and closer to the rim, Castelan said.

"The volcano becomes more dangerous as the crater fills with lava, and the domes that form are closer in elevation to the crater rim," Sheridan told The Associated Press in an email. "Explosions can more easily throw red hot lava fragments over the rim and onto the volcano flanks."

In 2000, Popo's floor was 150 yards below the rim of the crater, compared to 50 yards today, he said. In the case of Mount St. Helens, the summit slid away and a new crater was formed, according to the U.S. Geological Survey. Mount St. Helens' huge eruption came just 15 to 20 seconds after a 5.1 magnitude quake.

Castelan, a 42-year-old father of three, has worked in the laboratory since 1997 and steadily moved up to technical director. The job has meant days without going home, or tending to equipment failures on nights and weekends.

Sometimes he thinks, "Not again," when he's called while off duty. But he said he does the work gladly because he knows how important it is that the people in the shadows of Popo stay alerted and safe.

"It's a very important relationship that we've established," he said. "We take care of this volcano."