Geologists say the 26,000-square-mile blanket of pumice was probably formed when lava from an ocean floor volcano known as Monowai came in contact with seawater.
The seamount is located well northeast of New Zealand and about 225 miles southwest of Tonga.
Since the pumice was less dense than the water, it quickly rose to the surface and was eventually discovered by a naval aircraft about 650 miles northwest of Auckland, where it had drifted.
“The rock looked to be sitting 2 feet above the surface of the waves, and lit up a brilliant white color in the spotlight," said NZ Navy Lt. Tim Oscar.
“It looked exactly like the edge of an ice shelf.“ Oscar added that the layer of pumice is far too light to pose a significant threat to maritime navigation even though it is 250 nautical miles long and 30 nautical miles wide.
Photo: Tim Oscar - NZ Navy