등잔 밑이 어둡다 (Deungjan mit-i eodupda)

Literally: It's dark under the lamp

Example: “안경을 한 시간이나 찾았는데 머리 위에 있었다. 등잔 밑이 어둡다.” — I looked for my glasses for an hour and they were on my head, right under my nose.

English equivalent: Right under one's nose

Origin: A deungjan is an old oil-lamp dish on a stand: it lights the room but casts a shadow directly beneath itself. The traditional saying means that what is closest is often the hardest to see. Japanese has the same image, tōdai moto kurashi.