I HAD ARRIVED IN PASTO, Colombia, the week before to join an international team of more than 100 fellow scientists studying the Galeras volcano. Old friends and colleagues streamed in for our convention, a conference I would run with Pasto native Marta Calvache, the director of the Colombian government's geological observatory here and my close friend and prized graduate student. We had worked together in Pasto, and on Galeras, for years.

The city of Pasto (pop. 300,000) sits at 8,400 feet in a wide green bowl. Its central square is only five miles from the crater, and on a clear day residents can see steam rising from the squat, barren volcano, its apron stained green, brown, and gold by a patchwork of crops. The mountain's lower realms are thick with white-flowering coffee bushes, yellow-flowering guava trees, red and purple bougainvillaea. Galeras seems to be a generally benign presence; Pastusos are quick to point out that, despite being the most active Colombian volcano in recent centuries, Galeras had never killed anyone in recorded history.

But Galeras had grown more threatening. In 1988, after slumbering for 40 years, the volcano awoke with a series of minor earthquakes. In March 1989 it coughed up a cloud of ash that fell on Pasto. Two months later, an eruption sent a plume two miles high, sprinkling dust on the surrounding towns. Then, in August 1991, this activity rose sharply. By November a hardened lava dome was being squeezed up out of the volcano, eventually growing to a height of 150 feet. On July 16, 1992, the dome was blown to pieces, catapulting 12-foot boulders throughout the amphitheater and sending a column of ash 3.5 miles into the air.

By the time we arrived in Pasto six months later, Galeras seemed like the perfect specimen—active, but quieter than it had been in four years. Only a faint twitch of seismicity occasionally etched a tornillo, a screw-shaped signal, onto the black seismograph drums in the geological observatory. Sadly, my colleagues and I failed to appreciate the significance of these subtle signals.