The Mount Vernon Tunnel

The tunnel exists, in this film, beneath the cellar level of the Mansion. (Mount Vernon does have basements, but no tunnels of the kind shown.) Inside the tunnel is a brick recess and, set into the recess, a small hand-blown bottle full of clear water. Sprinkle the water on a particular ledge of stone — the film's conceit is that this is the only solvent that activates a residue painted there by someone two and a half centuries ago — and a figure resolves: an eagle with a scroll. Symbol of Cibola. Symbol of the seal we already have. Two clues confirm the same destination, which is what good cipher-making always does: redundancy until the reader trusts the answer.