Bio-inspired robots are an awesome idea, since they take aspects that evolution has been refining for however many bajillions of years and put them into practice giving robots new capabilities. Northwestern University has created a robot called GhostBot modeled on the black ghost knifefish, which uses one single horizontal fin to propel itself forwards, backwards, and even straight up:
Pretty cool, right? Here's how it works:
Observations revealed that while the fish only uses one traveling wave along the fin during horizontal motion (forward or backward depending on the direction on the wave), while moving vertically it uses two waves. One of these moves from head to tail, and the other moves tail to head. The two waves collide and stop at the center of the fin.
The team then created a computer simulation that showed that when these “inward counterpropagating waves” are generated by the fin, horizontal thrust is canceled and the fluid motion generated by the two waves is funneled into a downward jet from the center of the fin, pushing the body up. The flow structure looks like a mushroom cloud with an inverted jet.
To get a sense of the potential of this kind of mobility system, check out a video of the actual fish:
That's a pretty impressive adaptation, if you ask me, and effectively puts conventional thrusters to shame.