A Noise Like Thunder

Dawn broke with clarity. Even the encircling, snow-topped Sierra Madre was etched sharply in the distance against an azure sky. This was unusual for Mexico City, which is normally shrouded in a brown smog generated by the exhausts of some 3 million cars. But in just four earthshaking minutes, starting at 7:18 a.m. last Thursday, the day's auspicious beginning turned into a nightmarish disaster--and the bright skies only illuminated the extent of the tragedy.

"The noise was like thunder," recalled Tito Garcia Mendez, 60, who was riding the city's sleek subway, one of the capital's brightest successes. "All the lights went...