Researchers at Northwestern University (including a robotics expert who consulted for Tron: Legacy) have developed a robotic knifefish, inspired by the actual fish that swims in the Amazon. The team of professors and grad students had only seen the living black ghost knifefish (how's that for a name?) swimming horizontally, until one day they witnessed the fish swimming vertically, and were thus inspired to find out how this fish could move upwards in its tank.
By creating two "inward counterpropagating waves" along its fin, the fish creates a downward jet of fluid, which pushes the fish vertically. To visualize the flow created by this downward jet (which, according to the researchers, "looks like a mushroom cloud with an inverted jet"), the researchers shone a laser sheet into the water in order to track individual particles.
After observing the actual fish in action, the Northwestern Team and Kinea Design developed a robotic fish with 32 motors that control an artificial fin made out of Lycra. To the researchers' amazement, the robotic knifefish worked on the first try, swimming upward in a tank at Harvard University. To see more of this robotic knifefish in action, check out the video: