Researchers at the University of South Florida have discovered that jellyfish are the world's most efficient swimmers due to the way they generate a 'ground effect' force in the water. This force is created by two vortex rings that spin in opposite directions beneath their translucent bodies, acting like a 'virtual wall' that resists and propels the jellyfish forward. This finding, published in Proceedings of the Royal Society B, has implications for the development of underwater vehicles. The research team observed that jellyfish can increase their swimming speed and distance by 41% and 61% respectively, by using this ground effect force, without producing cavitation bubbles or noise.