So big that a floating raft of pumice stone in the South Pacific Ocean, that happens to be 300 miles long and about 35 miles wide, or roughly 200 square miles larger than New Jersey, could go undetected for any length of time.
It’s likely the rocks are the result of an underwater volcano, Monowai, erupting. Lava turns to pumice when cool rapidly. That also means it’s possible an underwater volcano has been erupting without scientists knowing either. [Thanks, Paul!]