Why Nobody Knows How Much Ocean Is Unexplored

Ninety percent of the ocean is unexplored. The line is on museum placards, in documentary narration, and in at least three U.S. Senate committee reports. It has never come with a citation.

This paper follows it back to where it starts. The trail ends at a presidential panel report from 2000 whose only internal support for its own headline number is a copy of the memorandum that ordered the report. Then it measures what can be measured: 28.7 percent of the seafloor mapped to modern resolution as of April 2026, and an upper bound of 0.001 percent of the deep seafloor visually observed. The sentence turns out to be true, false, and a serious understatement, depending on which of three quantities you take it to mean, and those three differ by four orders of magnitude.