When instinct orders them to swim upstream to Lake Burragorang, juvenile eels, known as elvers, are faced with the 142-meter-tall wall of Warragamba Dam. Not a problem. They improvise with a detour that finds them working uphill along an adjacent stream bed, gaining access to a drainage ditch, and squirming along an underground culvert before reaching the serene lake. That's the successful end of a thousands kilometers long trip from the Coral Sea, which takes more than one year.