Popocatepetl means smoking mountain in Aztec, and the eponymous volcano is living up to its name.
Authorities are closely eyeing Popocatepetl volcano, which sits just 50 miles from Mexico City, and building shelters for residents who may have to flee for their lives.
The volcano has been spewing steam since Saturday night, and its eruptions have gotten continually stronger.
In ancient times Popo was known to bury entire Aztec settlements.
Now, after sitting quietly for 50 years, it has been emitting ash almost daily, and big explosions intermittently, since 1994.
The eruptions started strengthening two weeks ago and have been increasing steadily since Saturday.
VolcanoDiscovery.com reports "a relatively long phase of strong volcanic tremor today and high levels of SO2 output suggest that the rise of magma is accelerating. A continuous plume of steam and some ash was observed last night reaching 1 km high, in addition to intermittent explosions. Lava glow at night is now strong."
Mexican soldiers and federal police on Sunday closed off a seven-square-mile (18-square-kilometer) zone around the cone of the 17,886-foot (5,450-meter) volcano.
Webcams show large chunks of molten rock spewing from the crater, and ash has rained on nearby Puebla. The National Center for Disaster Prevention elevated its warning level to Yellow Phase 3, the fifth stage of a seven-stage warning scale.
The next stage, Red Phase 1, would bring a voluntary evacuation.
Four German tourists and their guide were killed last week when Mount Mayon exploded in the Philippines.
By Cheryl Rosen
