What struck the statue in Nebuchadnezzar's dream and became a great mountain?

Answer: A stone cut without hands

A stone cut out without human hands struck the statue's feet, shattering it, and then grew into a mountain that filled the earth.

“As you watched, a stone was cut out, but not by human hands. It struck the statue on its feet of iron and clay, and crushed them.”— Daniel 2:34 (BSB)