Now, what's that for?
I bet you the money IS under the rock.

I'm going to dig in the clay." "That ain't no bad notion, Tom!" said Huck with animation.
Tom's "real Barlow" was out at once, and he had not dug four inches before he struck wood.
"Hey, Huck!--you hear that ?" Huck began to dig and scratch now.

Some boards were soon uncovered and removed.

They had concealed a natural chasm which led under the rock.
Tom got into this and held his candle as far under the rock as he could, but said he could not see to the end of the rift.

He proposed to explore.