The Secret Garden
As she looked she saw something almost buried in the newly turned soil. It was something like a ring of rusty iron or brass, and when the robin flew up into a tree near by she put out her hand and picked the ring up. It was more than a ring, however; it was an old key which looked as if it had been buried a long time.
Mistress Mary stood up and looked at it with an almost frightened face as it hung from her finger.
‘Perhaps it has been buried for ten years,’ she said in a whisper. ‘Perhaps it is the key to the garden!’
–The Secret Garden, by Frances Hodgson Burnett
I’d venture to guess this old key has been buried more than 10 years; it was fun to find it while turning the soil for fall plantings.