Deep-diving manned submersibles, such as Alvin, which gained worldwide fame when researchers used it to reach the wreck of the Titanic, have helped advance deep-ocean science. But many scholars in this field have noted that the number and capabilities of today's underwater vehicles no longer meet current scientific demands. At the same time, the relative value of manned and unmanned vehicles is often disputed. The report finds that new submersibles -- both manned and unmanned -- that are more capable than those in the current fleet are needed and would be of great value to the advancement of ocean research.