Many astronauts returning to Earth after long-duration missions in space suffer from blurry vision that does not always get better. Now, after years of speculation and study, researchers believe they have finally isolated the cause: The liquid sloshing around the brain is building up in places it shouldn’t, squishing their eyeballs until they permanently flatten.

The condition is called visual impairment intracranial pressure, and it afflicts almost two-thirds of the astronauts who have spent extended periods of time aboard the International Space Station. (Also see "Astronauts' Fingernails Falling off Due to Glove Design.")

NASA first identified the mysterious syndrome in 2005, when astronaut John Phillips’s vision went from 20/20 to 20/100 after six months in orbit. Extensive physical