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.
Underwater, light transmits data faster than radio waves, but it fades so quickly that the signal reaches only a couple of meters.