An iceberg is a piece of freshwater ice more than 15 m long that has broken off a glacier or an ice shelf and is floating freely in open water. Smaller chunks of floating glacially-derived ice are called "growlers" or "bergy bits". The 1912 loss of the RMS Titanic led to the formation of the International Ice Patrol in 1914. Much of an iceberg is below the surface, which led to the expression "tip of the iceberg" to illustrate a small part of a larger unseen issue. Icebergs are considered a serious maritime hazard. Icebergs vary considerably in size and shape. Icebergs that calve from glaciers in Greenland are often irregularly shaped while Antarctic ice shelves often produce large tabular icebergs. The largest iceberg in recent history, named B-15, measured nearly 300 km × 40 km.