Hearing Voices

 is a Fallout 3 quest that is added to the game with the Point Lookout add-on.

Walkthrough
After speaking with Nadine and getting the key to unlock Jackson's sacred praying grounds beneath the Wrecked Seatub, return to Desmond and tell him the news. Of course, Desmond "chastises" you for not interrigating the Tribal's leader first. Make your way to the Seatub and enter the caverns.

The path to finding Jackson is easy enough, follow the torches to his inner sanctum. Although there are Mirelurks in the caves, you can sneak your way through without fighting them. Upon entering the final room, you find Jackson kneeling before a table speaking to...a talking brain? Let Jackson and the Brain finish, then speak with the Brain. It reveals itself as a hologram of Professor Calvert, or what remains of him, who is using the local tribal cult as means to destroy a signal jamming device that Desmond is using to restrict his broadcasting signal. Although his is rather disappointed that the tribals take his instructions as destroying his ancestral mansion instead of the jamming device.

Report back to Desmond with the info.