Your body is lined with a network of fluid-filled cavities that—until now—were unknown to science. The team that made that discovery thinks the cavities qualify as a new human organ, which they’ve dubbed the “interstitium,” Live Science reports. The new organ was spotted when researchers looked at live human tissue with a new imaging technique. Previous methods have mostly looked at tissue that’s dead and drained of fluid, so the cavities weren’t visible, the team reports in Scientific Reports. Anatomy textbooks may need revising if other researchers agree that the interstitium is, in fact, a newly identified organ.