'caldera' is a large crater-like feature formed after a volcano eruption, according to National Geographic.
When a volcano erupts, magma is expelled forcefully from within the magma chamber underneath the volcano. Because the structural support that the magma chamber had provided the volcano disappears, it causes the sides and top of the volcano to collapse inward. Calderas often vary in size ranging from 1 to 100 kilometers in diameter.
There is also a type of caldera called 'resurgent caldera.' These occur when large magma chambers underneath the volcano empty in such a way that causes a series of pyroclastic flows, fast moving current of hot gas and volcano matter. Over time, the magma chamber is refilled and the caldera floor is pushed upward, thus the name 'resurgent' which means "risen again."
So, what was discovered?
The marine geophysicists group that discovered the existence of the world's largest caldera consists of New Zealand-based Filipino scientist Jenny Anne Barreto, along with co-authors Ray Wood and John Milsom.
The caldera named 'Apolaki,' which means "giant lord," is based on the Filipino mythical god of the sun and war.
The University of Philippines Marine Science Institute states in a Facebook post that Apolaki Caldera has a diameter of roughly 150 kilometers, more than twice the size of Wyoming's Yellowstone Caldera in the U.S. which is only about 60 km.