City Life: Weathermen who intercede with spirit of the volcano

YOU CAN'T help but feel a bit edgy caught in the great sprawl of Mexico City. Every public building here shows an evacuation route, in case some geo-cataclysm comes shuddering through the surrounding cordillera and sparks off an earthquake or eruption. All the highways leading in or out of the capital are marked with a volcano alert code. Since I arrived, it's constantly shown the yellow caution signal.

Scratch the surface just a bit and Mexico is seething. There are so many old volcano cones on the valley floor that it looks like the aftermath of some gargantuan Guy Fawkes' celebration, and quite a few are still smouldering.

Over near Colima, when lava started oozing down the sides of the Volcan de Fuego last week, all villagers were evacuated, but came defiantly back to till the rich soil as soon as the smoke settled. There is such concern about when Popocatepetl, the 17,500ft volcano looming on Mexico City's outskirts, might blow its top that the university website with daily updates on its activity gets more hits than a cyber-sex chatroom.

The last huge eruption was 1,200 years ago. Scientists feel the next one could be imminent, with catastrophic effects on the 30 million inhabitants of Mexico City and 16 surrounding villages. With remote space gadgetry, they hope to foretell big upheavals of ash and rubble, and the subsequent deadly mudslides and quakes, well in time to save lives. Similar equipment predicted a trembler in California a fortnight in advance.

Enrique Cabal, a vulcanologist from Mexico's public university, Unam, and Professor Tim Dixon, a former Nasa scientist, recently hiked up to plant new sensors on the sides of Popocatepetl and to sweep the debris off the solar panels that power them.

Rather foolishly, I joined the work party for a high-altitude trek that was like a breathless trudge up a down-escalator. My heart was beating like a hummingbird's, though I wasn't toting anything heavier than a pad and pen. I never expected it to be chilly on a volcano that is technically in the tropics, but winds came whipping off the glacier near the top. Since it was coated with grey ash, I hadn't noticed it glittering under the clouds. While I squinted to make out the icy patch, Popocatepetl belched three times, emitting thick columns of water vapour, sulphur dioxide, and grit. Deep-throated thuds sounded far away. Unexpected volcanic eruptions can force ash plumes unpredictably high into the stratosphere and have been known to shut down jet engines on unlucky airliners that cruise by.

"At this distance, there's not much danger," Professor Dixon assured me. "Of course if you walk right up to the crater you can get hurt or killed." His theory is that before blowing its top, a volcano inflates due to increased molten rock inside. Global positioning satellite technology can measure how much the mountain is expanding.

Radio transmitters now in place on Popocatepetl's slopes, one kilometre from the summit, can signal such changes round the clock. So far this year, the mountain has shifted some eight centimetres - whopping by geological standards.

An emergency escape circuit is clearly marked out on the deserted paths down Popocatepetl by Senapred, the federal agency, which posts hoardings, with graphic sketches for the illiterate. Police check our passes and wave us on up with the scientific team to pick our way through fresh pumice chunks scattered on the slopes. The peak has been officially closed since 1993, but four Mexican hikers were killed in 1995, sneaking up past an army blockade to videotape inside the caldera. Dona Maria de los Angeles, whose niece died in that violent outburst, often recounts how officials found the girl "after the volcano vomited ashes and stones, with her hands outstretched as if she wished to stop that big rock that crushed her head". The four bodies eventually were retrieved, riddled with holes like shotgun blasts.

Some of the highland peasants feel disquiet when scientists and the army trample over "Don Gregorio", their fiery mountain. Old-timers still shudder over how in 1919, hare-brained engineers tossed dynamite down the crater to extract sulphur and triggered potent quakes for the next year. Since Aztec times, certain local dreamers always sense portents of volcanic explosions and electric storms and so are expected to redirect hailstorms while they sleep. With great reverence, these elderly tiemperos (weathermen) erect wooden crosses on hillsides in sight of volcanic craters, in order to show the cardinal points to any person ever killed by lightning. Colourless flowers - never red, which would encourage heat - and alcohol are offered to soothe any unsettled ghosts at the start of the May rains.

In trance, the tiemperos seek out the volcano's spirit and report back whether or not it feels wrathful enough to fume. With these tiemperos interceding, loss of life has been kept low on Popocatepetl for centuries. Compared to Tim Dixon, whose geological concept of imminent ranges from a week to 750 years, their predictions are quite precise.

Jan McGirk