solid ground

Solid ground is earth or land that is firm and stable underfoot, as opposed to water, mud, or unstable terrain.

The phrase often appears in contexts of relief or safety: After the rocky boat ride, I was glad to be back on solid ground.

Figuratively, solid ground suggests a position of certainty or security, especially in arguments or negotiations: The data puts us on solid ground.

To be on shaky ground is the opposite—uncertain or vulnerable.