A chunk of ice four and a half times the size of Manhattan fell off an Antarctic glacier. This marks the second time in two years that a huge iceberg has calved from the area. The break-off could speed up the loss of more ice, eventually eating away at a barrier that prevents land-based ice from flowing to the sea.

The Pine Island Glacier is located in West Antarctica, and has been one of the largest contributors to sea level rise on the continent, since it’s been losing ice at an accelerating rate over the past 40 years. In 2015, a 224-square-mile iceberg calved from the glacier. The latest cleavage, which was photographed by satellites over the weekend, has resulted in an iceberg about 103 square miles in area. It was the fifth big calving event since 2000, says Stef Lhermitte, a satellite observation specialist at Delft University of Technology in the Netherlands.

These chunks of ice are breaking off a floating portion of the glacier, so they don’t raise sea levels by themselves. Floating ice shelves are like ice cubes in a glass of water: when the ice cubes melt, the water level in the glass doesn’t rise. But the floating ice shelves do create a barrier, which keeps the ice that’s hanging out on land from flowing into the sea. Without the barrier, the extra water from the continent could mean higher seas. Pine Island Glacier alone could raise sea levels by 1.7 feet if it all melted, according to The Washington Post. That’s why scientists are keeping an eye on it.

.@NASAEarth 's MODIS satellite gives an overview of #PIG's glacier front location since 2009 + corresponding 3 major calving locations 7/n pic.twitter.com/miP1RAUCBF — Stef Lhermitte (@StefLhermitte) September 24, 2017

Icebergs calve off Antarctica all the time, but the chunks of ice that broke off Pine Island Glacier are somewhat unusual. The latest iceberg, for instance, cracked off from the center of the ice shelf rather than the sides. That could be because warmer ocean waters are eating away at the ice from beneath, weakening the glacier, The Washington Post says. Following the recent break-offs, it looks like the glacier is retreating.

Scientists will keep monitoring Pine Island Glacier, as well as other ice shelves that are producing some of the biggest icebergs ever created. The whole of West Antarctica alone might contribute 10 feet of sea level rise. So if the trend continues, that could spell doom for cities like New York and Miami.