Northernmost of the sizeable ice shelves of Antarctica, the Larsen shelf is located in the north-western part of the Weddell Sea and off the east coast of the Antarctic Peninsula, roughly between latitudes 64°S and 68°S. The small northern part is called Larsen A while the main part, further south, is Larsen B. It was named after Norwegian whaler C.A. Larsen who sailed along its coast in 1893. It's believed to have been formed fairly recently, possibly as little as 20.000 years ago.
Larsen A has shrunk to virtually nothing following a massive collapse in 1995 which is generally believed to have been caused by climate change. The larger Larsen B shelf was originally 12.000km² but is not in much better shape and has shrunk significantly. Shortly before that a massive iceberg had calved from Larsen B. Other calving events in 1986 (the largest known at the time) and 1998 indicate the extent of warming in that particular part of the world and are widely used as an argument in support of the global warming theory. Larsen B is expected to disintegrate before too long and the area is under intensive study.
Update, 2002-03-19: It was expected to last another 3-4 years but the Larsen B ice shelf has collapsed. The last remnant of it, with a size of approximately 3250 km² and 200m deep, gradually collapsed in the ocean during the final month of the Antarctic summer. According to one British Antarctic Survey scientist, "It is hard to believe that 500 million billion tons of ice sheet has disintegrated in less than a month." This will not affect sea levels since the ice was floating to begin with but will if the land ice cover moves into the ocean.
British Antarctic Survey
Earth and Sky radio
US Geological Survey