Vast volcanic 'raft' found in Pacific, near New Zealand
"A vast 'raft' of volcanic rocks covering 10,000 sq miles (26,000 sq km) of ocean has been spotted by a New Zealand military aircraft." - from BBC article linked above.
The floating rocks are pumice, a type of volcanic rock that's lighter than water due to gases trapped inside it. It's hard, but it looks like a stone sponge.
To put that in perspective, the island of Hawaii is only about 4,028 sq miles (10,430 sq km.) So it's more than twice the size of the island of Hawaii.