A lightweight robot modeled on diving birds can swim underwater, break through the surface, and continue flying without using feet or propellers. The feat exposes a difficult mechanical compromise between movement in dense water and thin air. Engineers at MIT and EPFL in Lausanne, Switzerland, built the flapping-wing aerial-aquatic vehicle, or FAAV, to explore how animals such as puffins, petrels, gulls, and loons move through both environments. The untethered robot weighs about 250 grams and resembles a small bird. It has a slender waterproof body, two flexible membrane wings, a battery-powered motor, and a steerable tail. Researchers can replace the wings and tail to test different dimensions and mechanical properties. Raphael Zufferey, left, and Moritz Hüsser work on their robot design. (CREDIT: John Freidah) Balanced design Experiments in tanks, a wind tunnel, indoor flight spaces, and Lake Geneva showed that one carefully balanced design could fly, swim, plunge into water, and launch back into the air. The results will appear in the journal Science. The work provides a robotic test bed for questions that are …