Giant squid have the largest eyes on Earth, about the size of a basketball—but why do they need such big peepers? So they can see a sperm whale in time to escape its gaping jaws, researchers report online today in Current Biology. Seeing under water is tricky because light fades out at deeper depths and water makes distant objects disappear even before they are too small to see. But eyes are expensive to build and maintain, so such huge ones must serve a purpose. In the new study, researchers developed a computer model to look at what different-sized eyes could see at different water depths. In the dark deep, sperm whales and other animals become "visible" because their movements disturb small organisms that give off light, similar to a flashing firefly. The modeling showed that basketball-sized eyes could detect whales amid flashes of light from more than 120 meters away—giving the squid enough of an edge to make a quick exit. Most organisms are not faced with such big predators, so they don't need to see far away and thus get by with smaller eyes.