Does anyone know the title of this young adult book?

Science fiction, from the 70's. I think it was a Scholastic book, not very thick. It's about people living in an authoritarian underground city in a mountain. Two bald-headed boys wearing white gowns find a crack in a tunnel wall that leads to the outdoors. Some adult tells them not to go into the crack, but they disobey. They follow the crack to the surface and...? I think it was illustrated. It was NOT "This Time of Darkness" by H.M. Hoover. It may have been by Ben Bova, but I don't think so.