This is a tricky question! There isn't a single, perfect opposite for "sky" because it has multiple meanings. Here are some options, depending on how you're thinking of it:

Based on physical location:

* Earth: The sky is above us, and the earth is below us.

* Ground: This is a more common word for the surface we walk on, the opposite of the sky.

Based on visual appearance:

* Darkness: If you're thinking of the blue sky as the opposite of night, darkness is a good choice.

* Solid: The sky is open and vast, while something solid is concrete and impenetrable.

Based on metaphorical meaning:

* Limitations: "Reach for the sky" is a metaphor for ambition. The opposite might be "Stay grounded" or "Accept limitations."

Ultimately, the best opposite for "sky" depends on the context of your question!