The gigantic cluster, made up of lightweight pumice expelled from an underwater volcano and spread over 26,000 sq kilometres, has been spotted drifting in the ocean by a military aircraft, about 1,000 kilometres off the coast of New Zealand.
A naval ship had to change direction to avoid the rocks, which were spread out over thousands of square miles.
Experts reckon the rocks were formed by lava released from an underwater volcano, the 'Daily Mail' reported.
Lieutenant Tim Oscar said that while he knew his ship the HMNZS Canterbury was in no danger from the floating rocks, or pumice, which is solidified lava filled with air bubbles, it was still the "weirdest thing" he's seen in 18 years at sea.
"As far ahead as I could observe was a raft of pumice moving up and down with the swell," he said.
Scientists aboard the ship said the pumice probably came from an underwater volcano called Monowai, which has been active recently.
They said the phenomenon was unrelated to increased volcanic activity in New Zealand this week, including an eruption at Mount Tongariro that sent an ash cloud 20,000 feet into the atmosphere.
The pumice is lighter than seawater - so the rocks quickly rise to the ocean surface.