iceberg
n. countablen. a massive piece of ice that has broken off a glacier and is floating in the ocean. Most of its size is hidden underwater, so you only see a small part of it.
n. a large mass of ice detached from a glacier or ice sheet and floating in open water. Typically, only about one-tenth of the total volume is visible above the surface.
The ship slowed down to avoid a large iceberg.
The research vessel tracked the massive iceberg as it drifted into warmer southern waters.
Oceanographers monitored the disintegration of the iceberg, noting how the melting process released trapped nutrients that temporarily boosted the local marine ecosystem's productivity.
Partial calque of Dutch ijsberg (compound of ijs (“ice”) + berg (“mountain”)), from Middle Dutch ijsberch. First used to describe a glacier as seen at a distance from a ship then used as a term to describe the floating chunks of ice broken off from such glaciers. Cognate to German Eisberg, Danish isbjerg, Norwegian isberg and Swedish isberg. Figurative senses in reference to the fact that only one-tenth of an iceberg is usually visible above water.
Often used metaphorically in the phrase 'tip of the iceberg' to describe a small visible part of a much larger problem.