Another huge iceberg breaks free from Antarctica.

In mid-March, the second largest iceberg ever recorded broke free from Antarctica's Ross Ice Shelf and began floating freely in the adjacent Ross Sea, according to the U.S. National Oceanic and Atmospheric Administration. Several days later, three smaller icebergs also crumbled from the Texas-sized shelf, renewing concerns about the disintegration of the southern ice cap.

The gigantic iceberg, dubbed B-15 after the Antarctic quadrant and sequence in which it was first sighted, measured some 11,000 square kilometers in area (300 kilometers long and 37 kilometers wide )--roughly the size of Jamaica.

Only one other iceberg, spotted in 1956, has been bigger. But the visible portion of B-15, which towers an estimated 30 meters above the water, is just "the tip of the iceberg," according to Matthew Lazzara, a senior research specialist with the University of Wisconsin's Antarctic Meteorological Research Center. Below the surface, the ice mass could plunge as deep as 400 meters.

Despite signs that rising temperatures may be causing accelerated ice melting elsewhere in Antarctica, the B-15 break up--or "calving"--is probably not a result of global or even regional warming. Ted Scambos, a scientist with the National Snow and Ice Data Center in Boulder, Colorado, explains that ice shelves--the vast overhangs created when land-based glacial ice flows out into the ocean and begins to float--normally shed large icebergs every 50 to 100 years, as part of their natural cycle of loss and re-growth. And while temperatures may be rising elsewhere on the continent, Scambos says, those around the Ross Ice Shelf have remained relatively constant, rarely climbing above freezing even in the summer months.

But if another large iceberg were to break from the shelf in the near future, this could be a sign of something more serious, scientists say. The March calving comes after several years of remarkable ice loss in Antarctica, some of which has been directly linked to regional warming. Air temperatures at the Antarctic Peninsula, a narrow arm of land that swings towards the tip of South America, have risen by some 2.5 degrees Celsius over the last 40 years. Three ice shelves there have completely disintegrated within the past decade, and two others--the Larsen B and Wilkins--have lost more than one-seventh of their combined 21,000 square kilometers since late 1998. Unlike the larger and infrequent calvings on the Ross Ice Shelf, these break-ups are characterized by the near-continuous shedding of thousands of smaller icebergs--an almost sure sign of melting.

Collapse of Antarctica's ice shelves alone would not raise sea levels, because these already float on the ocean's surface. But their continued break-up could fatally weaken the two larger ice sheets from which they extend--dense ice fields that hold some 70 percent of the planet's fresh water. The collapse of the smaller (and less stable) West Antarctic Ice Sheet would raise sea levels by an estimated 6 meters--enough to cause serious flooding of the world's coastlines--while loss of both sheets would raise them by as much as 70 meters.