A beam of light fades hundreds of times faster underwater than in air — like a flashlight in dense fog. The postman robot uses this to its advantage: it locates sensors scattered on the seafloor by their faint glow and swims right up. Then it's like a fuel delivery truck: if a sensor's battery is almost empty, it urgently 'refuels' it; if it's half-charged, it first collects data, then provides power; if fully charged, it simply exchanges data.

The most surprising part: the threshold of when to charge versus just read is calculated using a single simple formula. No complex algorithms — the robot simply checks the charge percentage and decides. Mathematicians proved that this 'common-sense' strategy — 'if below 20%, charge' — turns out to be optimal. Thanks to this, underwater networks can operate for years without human intervention, and the postman robot will use minimal fuel on its rounds.