Where is the secret key found in The Secret Garden?

In "The Secret Garden" by Frances Hodgson Burnett, the secret key to the locked garden is found by Mary Lennox when she discovers it buried in the soil while exploring the grounds of Misselthwaite Manor. The key symbolizes the unlocking of not just the garden itself, but also the emotional and personal growth of the characters. This hidden key leads to the transformation of the garden and the individuals who tend to it.