Many people assume that more and better science alone holds the key to solving ocean challenges related to climate change, but it may surprise you to learn that protecting oceans depends on ocean history. Understanding, or “fathoming” the interrelationships between people and oceans involves as much attention to imagination as to modern science. Helen Rozwadowski’s TEDx Boston talk looks at historical depictions of the ocean’s depths to discover past views of the oceans and the accompanying ambitions for using them. Decisions about future oceans hinge on our visions of the undersea, so attention to how those visions emerged holds one important key to effective ocean conservation.