The lads searched and researched this place, but in vain.

Tom said: "He said UNDER the cross.

Well, this comes nearest to being under the cross.

It can't be under the rock itself, because that sets solid on the ground." They searched everywhere once more, and then sat down discouraged.
Huck could suggest nothing.

By-and-by Tom said: "Lookyhere, Huck, there's footprints and some candle-grease on the clay about one side of this rock, but not on the other sides.