Ping Pong balls make plane buoyant Modern Mechanix, 1936.

Fun fact, from Donald Duck artist Carl Barks wikipedia entry…

A 1949 Donald Duck ten-pager features Donald raising a yacht from the ocean floor by filling it with ping pong balls. In December 1965 Karl Krøyer, a Dane, lifted the sunken freight vessel Al Kuwait in the Kuwait Harbor by filling the hull with 27 million tiny inflatable balls of polystyrene. Although the suggestion is often made, Krøyer denies having been inspired by this Barks story. Some sources claim Krøyer was denied a Dutch patent registration (application number NL 6514306) for his invention on the grounds that the Barks story was a prior publication of the invention. However no definite proof of this story is available. Krøyer later successfully raised another ship off Greenland using the same method, and several other sunken vessels worldwide have since been raised by modified versions of this concept. The television show MythBusters also tested this method and was able to raise a small boat.