Installed in 1998, Idiom by Slovak artist Matej Krén is made from roughly 8,000 books arranged into a circular tower rising about 20 feet high. Mirrors at the top and bottom create the illusion that the structure stretches on forever, while a narrow, teardrop-shaped invites visitors to peer inside the tunnel—evoking the feeling of slipping into another world. It’s also become a striking, highly photogenic spot for visitors eager to capture something memorable to share.