A Face Without Eyebrows

Look closely and you'll notice the Mona Lisa has no eyebrows or eyelashes. For years people assumed Leonardo left them out on purpose, but high-resolution scans suggest fine brow hairs were once there and have simply faded or been lost to old cleanings.

Leonardo never delivered the portrait to the patron who commissioned it. He carried the unfinished panel from Florence to France and kept refining it until shortly before his death in 1519, which is part of why it feels so endlessly worked over.