The caldera of Hunga-Tonga Hunga-Ha'apai is now 4km (2.5 miles) wide and drops to a base 850m below sea level.
Before the catastrophic eruption, the base was at a depth of about 150m.
It drives home the scale of the volume of material ejected by the volcano - at least 6.5 cubic km of ash and rock.
"If all of Tongatapu, the main island of Tonga, was scraped to sea level, it would fill only two-thirds of the caldera," Prof Shane Cronin from the University of Auckland, New Zealand, said.
