If the cruise industry had existed between 10,000 and 20,000 years ago, ships headed in and out of Miami would have had to dodge icebergs. As the last ice age waned and climate warmed, immense lakes of glacial meltwater that accumulated behind natural ice dams occasionally burst forth from the mouth of Canada’s Hudson Bay and the Gulf of St. Lawrence. When those iceberg-laden outburst floods—some of them carrying more than 1 million cubic meters of water per second and lasting several months—reached the open sea, they took a right turn and flowed south along the coast as far as the Florida Keys, a new study suggests. The torrent-driven icebergs, some of them hundreds of meters thick, plowed troughs in the sea floor all along the continental shelf (like those found in 170- to 380-meter-deep waters off the coast of South Carolina; one such berm-edged trough extends from lower left to top center of the image). Sea levels have risen more than 100 meters since most of these troughs were formed, which has helped preserve them from surface waves that could roil and smooth seafloor sediments. Whereas the troughs off South Carolina measure up to 100 meters across and 20 meters deep, those off the central Florida Keys (now found in waters between 215 and 280 meters deep) typically are no more than 50 meters wide and 5 meters deep—as expected, because the bergs would have melted to smaller size as they drifted south, the researchers report online today in Nature Geoscience. Some of the iceberg scours off Miami Beach, probably created by icebergs the size of those setting sail from Greenland today, lie less than 12 kilometers offshore.