Within the cistern, the group finds a colorful cross-section of human cultures enjoying the concert, all tinted red by the lights, and the hues of the cavern giving a hellish glow.

Langdon spots a sign reading “MEDUSA” and realizes that the eponymous figure was known as one of the “chthonic” monsters of Greek mythology, thus matching Zobrist’s poem. He follows the sign to a corner of the cistern decorated with a statue of Medusa sporting an inverted head. Langdon immediately recognizes from the background of Zobrist’s video that this spot is his planned ground zero.

Brüder descends carefully into the water, using a waterproof penlight to slowly search for the submerged balloon. Langdon watches from the boardwalk and spots watery footprints nearby, indicating that someone has climbed out of the pool recently.

Langdon spots a woman in a burka hiding in the shadows and realizes it is Sienna just as she bursts out and plows past him.

Brüder finally locates the spot where the balloon was meant to be, but it has already burst, and he realizes in horror that the virus has been released.

Near the exit of the cistern, Sinskey receives word of this from Brüder via walkie-talkie and that the doors must be sealed to have any hope of containing its release.