MEXICO CITY (AP) -- Immense plumes of ash and smoke billow from the
                  Popocatepetl volcano within view of millions of people. It rumbles often and
                  occasionally hurls out bursts of glowing rock.

                  But more than four years after the volcano began a new cycle of eruptions,
                  scientists and officials are ever so cautiously growing less nervous -- not that
                  anyone takes an explosive, 5,452-meter (17,925-foot) mountain for
                  granted.

                  Seismologists map its every quiver. Chemists study the gases and debris it
                  burps out. Television cameras and radar keep constant watch on the crater,
                  and a video image of the mountain is updated every minute on the Internet.

                  Trying to determine what occurs miles (kilometers) underground, geologists
                  from Mexico and the University of Miami are turning to the skies, expanding
                  a satellite monitoring system for the volcano.

                  All of that effort "has been able to give, little by little, a more complete and
                  realistic vision of the risks for those in civil defense," said Roberto Meli, head
                  of the national Center for the Prevention of Disasters, which is in charge of
                  monitoring Popocatepetl.

                  Some 20 million people live within 80 kilometers (50 miles) of the volcano.
                  That has attracted attention to Popocatepetl's threat, but Meli says it also
                  blurs the fact that a far smaller number of people living much closer to the
                  volcano are at greatest risk.

                  Popocatepetl is capable of immense eruptions -- the last occurred about
                  800 years ago. It covers the ruins of two earlier volcanos on the same site.
                  Far more common, however, have been bursts of smoke, steam and ash
                  recorded periodically since the 14th century.

                  Keeping watch on the volcano can be costly and tiresome. Gasping for
                  breath in the thin air, scientists struggle up steep, sand-like slopes to build
                  and maintain small monitoring stations near the fuming crater. Helicopters are
                  sometimes pressed into duty to ferry equipment. Airplanes are used to check
                  gas emissions.

                  "Nothing is enough," said Roberto Quaas, the volcanologist in charge of
                  monitoring Popocatepetl.

                  He said there are plans to install pressure meters to record the force of
                  Popocatepetl's outbursts and new monitors to measure expansion or
                  contraction of the volcano's surface.

                  At a recent conference of volcano experts in Mexico City, investigators
                  showed photos of solar cells on recording stations peppered by volcanic
                  debris -- and a several-yard-wide (several-meter-wide) hole blasted by a
                  piece of rock hurled from the crater.

                  In 1995, five hikers were found dead near Popocatepetl's crater, possibly
                  due to volcanic gases. Officials now warn people to stay at least 7
                  kilometers (4 miles) from the crater.

                  Most scientists believe that somewhere below the volcano is a chamber of
                  magma, fed by flows from deeper within the Earth and vented to the surface
                  at the volcano's crater.

                  Earthquake studies indicate the chamber is deep below the surface --
                  perhaps 10 kilometers (six miles) beneath the crater, much deeper than the
                  chambers of volcanoes like Mount St. Helens in the northwest United
                  States.

                  If a chamber is shallow, a relatively small increase in pressure "could really
                  weaken the walls and cause an immense explosion, such as occurred at
                  Mount St. Helens," said seismologist Carlos Valdes of Mexico's National
                  Autonomous University.

                  The chamber beneath Popocatepetl "gives us security. If it is found further
                  below, the pressure of the weight of the rock keeps it stable," he said.

                  Valdes' monitors also show a remarkably small number of earthquakes deep
                  within the volcano -- indicating that when magma flows, it seems to do so
                  without meeting much blockage, so there may be less buildup of pressure.

                  Three Mexican states and Mexico City have mapped out plans to rapidly
                  evacuate hundreds of thousands of people if the volcano some 65 kilometers
                  (40 miles) southeast of Mexico City shows signs of a more dangerous
                  eruption.

                  Residents of nearby villages have refused suggestions to relocate. Some of
                  the Indian residents worship the volcano, yearly hiking up its slopes to a
                  dark outcrop known as "the bellybutton" to leave offerings to "Don Goyo,"
                  an affectionate and respectful nickname.

                  A large eruption could lead to fast-moving mudslides of volcanic ash that
                  could swamp parts of some towns. Much of the sloping terrain to the east of
                  the volcano is formed from such debris.

                  A huge, explosive eruption of the sort the volcano experiences every
                  millennium or so could cause a blast of volcanic material that would destroy
                  almost everything in its path.

                  Some villages were briefly evacuated in 1994 shortly after the volcano began
                  its latest eruption cycle.

                  Mexican volcanologist Servando de la Cruz said that with what scientists
                  know now, that evacuation might have been avoided. Officials fear people
                  will lose confidence in emergency measures if evacuations seem
                  unwarranted.

                  "Thanks to that understanding, the management of the situation has been a
                  little different," he said.

                  What would alarm him? "An activity completely different from what we have
                  seen until now," something that might leave investigators baffled about what
                  the volcano is doing, and what dangers it might pose.

                    Copyright 1999 The Associated Press.