Some 3,500 years ago, an event of cataclysmic proportions rocked the Mediterranean. The volcano at Thera (later known as the Greek island of Santorini) exploded with what is estimated at four to five times the eruptive force of Krakatoa in 1883, blowing a hole into the Aegean isle and sending out shock waves that, according to historians, would reverberate for centuries to come. The great seafaring Minoan civilization, the dominant Greek culture of the time, potentially withered away after clouds of ash enveloped its cities and great tsunami waves smashed its fleets. Stories of a world-shaking eruption linger in legends across the Mediterranean. For years, adventure-seeking archaeologists have even pored through Thera's geological record in search of the fabled lost city of Atlantis. Ancient Egyptian stela from roughly the same era chronicle a volcanic storm that "caused darkness in the Western region" and "annihilated" towns and temples alike. And some biblical scholars have even suggested Thera's destructive effects underlie the Old Testament's tales of God-sent plagues and devastation.

The eruption was small — in volcanic terms, that is — producing only about 3% of the ash ejected by Mount St. Helens in 1980. Instead, it was the mudflows that made Colombia's 1985 Nevado del Ruiz explosion the second deadliest in the 20th century and the fourth deadliest in recorded history. The volcano has been blowing its top since the Pleistocene era and has erupted three times in modern history, including in 1595 and 1845. But on Nov. 13, 1985, a relatively small explosion unleashed floods that swept away 1,500 people on one side of the mountain. On the other side was the town of Armero, the site of the worst destruction. 25-m.p.h. lahars (volcanic mudflows) obliterated the town and blanketed it in gray muck. When the landslides subsided, 23,000 people had died and damage was estimated at $1 billion — one-fifth of Colombia's GNP at the time.