Balsa, a swift-growing hardwood, contains mostly air.
In Ecuador's Andes Mountains grows a towering hardwood tree that matures in less than a decade. A man can carry a 30"-diameter log from this tree on his shoulder. A board foot sawed from that log, afloat in water, will support more than four times its 8-ounce weight.
Spanish colonists settling Ecuador in the 16th century named the remarkably light and buoyant wood balsa, meaning raft. And for generations before it was sold commercially throughout the world, Ecuadorians lashed balsa logs into rafts to transport goods to market.
Growing wild or on plantations, a balsa tree begins as a pinhead-size seed. Spurred by the equatorial climate, it shoots up to an 80' height and a 30" diameter in five to seven years. Strangely, balsa accomplishes this rapid growth without sapwood, relying instead on the pith to carry nourishment.
Loggers must fell mature balsa trees at once. If left to compete with surrounding vegetation, they form a tap root similar to a cactus. In a bizarre twist of nature, the world's most buoyant wood becomes saturated with water, making it commercially worthless!
Before inflatable gear became available, balsa was used extensively for lifesaving flotation devices on ships, because the wood contains about 92 percent dead-air space. Now, it's sold for making model airplanes, for insulating freight cars, and for shock-absorbing packing.