What you're looking at is a huge mass of pumice--volcanic rock that can float. Aircraft and naval vessels from New Zealand are studying the 10,000-square mile body which is likely the product of a recent underwater volcanic eruption:
The stretch of golf-ball-size pumice rocks was first spotted this week by a New Zealand air force plane about 1,000 kilometers (620 miles) northeast of Auckland. The rocks stretch for about 26,000 square kilometers (10,000 square miles). [...]
Scientists say the rocks likely spewed up in an eruption by an underwater volcano.