Final answer:
Ballard learned about the implosion of submarines at great depths, which helped him in his search for the Titanic.
Step-by-step explanation:
While exploring two sunken submarines, Ballard learned that the extreme pressure at great depths did not crush the submarines but caused them to implode as the air inside compressed. This knowledge helped him understand how the Titanic, which also sank at a great depth, likely imploded due to the pressure. It supported his theory that the Titanic's hull had not split open, and instead helped him locate and identify the debris field of the Titanic on the seabed.