Ride the Slime Mold Express!

From Science:

Slime If you want to design a railway system, you could do worse than hire a slime mold. Researchers have shown that, when grown on a map of Japan, the gelatinous, funguslike organism connects points of interest in a pattern similar to Tokyo's train network. Engineers might be able to take a cue from the organism's approach to design more-efficient transportation systems.

The trick has to do with how slime molds eat. When Physarum polycephalum, a slime mold often found inside decaying logs, discovers bacteria or spores, it grows over them and begins to digest them through its body. To continue growing and exploring, the slime mold transforms its Byzantine pattern of thin tendrils into a simpler, more-efficient network of tubes: Those carrying a high volume of nutrients gradually expand, while those that are little used slowly contract and eventually disappear.

More here.