An iceberg nearly 50 miles long and 24 miles wide is presently shaping a course between Cape Horn and the Antarctic Peninsula. The giant berg, called B-10A, started its voyage in 1992 when it drifted off a shoal by Antarctica’s Thwaites Glacier at about 110° W, 73° S. It then coasted westward into the Amundsen Sea and finally northeastward through the Bellinghausen Sea to arrive at its present position in Drake Passageabout halfway between Antarctica’s northernmost point and South America’s southern one. Total drift distance has been more than 1,700 nautical miles. B-10A, which is more than three times the size of Manhattan, began life as an iceberg known simply as B-10, but it fractured in two parts in June 1995 and then took two names, B-10A and B-10B. The latter melted away in 1997 to a more ordinary-sized berg that the National Ice Center does not bother to track. Bergs are given their alphanumeric names by region and chronology. Antarctica’s quadrants, 0° to 90° W, 90° to 180°W, 180° to 90° E, 90° E to 0°, are A, B, C, and D. Therefore, the 10th “mega”-iceberg to calve in the Amundsen and Eastern Ross Sea (since NIC began keeping records) was called B-10. “This is an enormous berg, extremely thick, that has survived a long time. It is so large that if you were standing on it, it would stretch to the horizon in all directions and you’d have no idea you were actually in the middle of the ocean,” said Ted Scambos, research scientist for the National Snow and Ice Data Center at the University of Colorado in Boulder. “However, some grizzled veteran sailor of the southern waters might notice something called ‘water sky,” in which the shading of high clouds and the atmosphere itself can reveal the presence of dark ocean water beneath, even at great distance. Scott, Cook, Amundsen, and the great aviators all knew this trick for navigating through icy waters.” Since icebergs of this size are carefully tracked by the National Ice Center, they pose little danger to commercial or recreational vessels. B-10A would appear very clearly on surface radar, Scambos said. “It is actually the smaller chunks of ice, like those that break from Greenland and Alaska, that present the most hazard to captains at sea, since they are so difficult to pick up by radar,” he said. “Global warming is unlikely to be a factor in the formation of these mega-icebergs,” Scambos said. The real indicator of climate change is the events observed in ice shelves on the northern tip of the Antarctic Peninsula. There, centuries-old ice shelves are crumbling away and not being replaced. “The northern limit of ice in Antarctica is receding,” he said. B-10, which is approximately 150 meters (495 feet) thick and probably started out at twice that, will continue its eastward course in the so-called West Wind Drift, a current/wind phenomenon that rotates around Antarctica clockwise at about 55° S. Now that it is in warmer, open water, it will perhaps last another year before fracturing and melting, Scambos said.