Iceberg B-10A, measuring 24 by 48 miles, is adrift between South America and Antarctica. It might endanger ships as summer approaches in the Southern Hemisphere and smaller chunks break off from the parent berg.

Satellite radar imaging -- which can penetrate cloud cover -- has been crucial in keeping tabs on the iceberg, notes Navy Lt. Andy Ulak, an official at the National Ice Center, an agency run jointly by the U.S. Navy, the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Coast Guard. "When you're operating near the poles, you get cold air over warmer water," says Lt. Ulak. "You get serious cloud formation."

The National Ice Center, based in Suitland, Maryland, monitors ocean ice conditions worldwide. The agency uses NOAA and military satellites to capture visible-light, infrared and microwave images. The center also acquires radar images from the Canadian Space Agency (including the image accompanying this article, which shows iceberg B-10A on a cloudy day when the berg was invisible to other types of sensors). NASA's QuikSCAT satellite, which monitors wind speeds and directions, also has helped track the iceberg's recent movements.