Now scientists believe they have finally discovered why dolphins are able to swim so fast – they have flippers that provide a thrust of 212lb.
Zoologist James Gray calculated the drag dolphins must overcome to swim faster than 20mph an hour.
He said dolphins lacked the muscles to swim so fast, and yet they did. This became known as Gray's Paradox.
Gray theorised that their speed possibly had something to do with their skin. Over the decades, scientists found flaws in Gray's work, and most biologists have rejected his theory.
Now a team of US scientists has used sophisticated underwater video to measure the power of a dolphin's tail.
They calculate 212lb of thrust – more than triple what a top Olympian like Michael Phelps can produce and enough to swim with the zip that confounded Gray seven decades ago.
"There is no paradox. The dolphins always had the muscles to do this," said Frank Fish, professor of biology at West Chester University in Pennsylvania. "Gray was wrong."
Mr Fish worked with Rensselaer Polytechnic Institute professor Tim Wei, who uses digital video and millions of tiny bubbles to study the complex movement of water stirred up by swimmers.
Computers track the bubbles' movement, making the invisible flow of water visible. He has used the technique to help US Olympic swimmers get the most from their stroke. Prof Wei presented his findings yesterday in San Antonio at an American Physical Society conference.