"Dig deep" is definitely a phrase to mean the opposite of "scratching the surface."
"Dig deep" is to go farther, try harder. A phrase like "scratching the surface" is obvious in it’s meaning the same way. It means to not go very far or not to put in too much effort. Or a beginning of sorts.
A bell went off today about my work and the future of it … DIG DEEP.
Just when i think i went far enough … time to dig deeper.