After all 30 of the Submarine Pieces are collected, the player must return to the Sonar Collections Dock and talk to Abigail to complete the mission. Abigail will thank Michael for his help and start getting a little nervous when Michael asks some questions about a possible sabotage that caused the death of her husband. She will give $10 and an autographed photo of her ex-lover to Michael for his hard work before leaving. After the cutscene ends, the player will have the choice to either kill her or let her go. The player's decision will have no consequences to the game.
- Bring the submarine parts back to Abigail.
- Kill or spare Abigail's life.
- A signed photo of Frank Mathers. Michael throws this into the sea at the end of the cut-scene and it's not seen again.
The mission "What Lies Beneath" starts at 26:31.
- Abigail's car is an Asea, one of the rarest cars in the game (PS3/XB360).
- The $10 received for completing this mission is the lowest monetary reward in the entire series.
- Michaels private car will spawn on the dock during this mission. This must be some kind of a joke, or glitch, because the car will be stranded here.
- When the mission ends, there will be instructions on screen to view mission completion details. This function is glitched and the instructions have no effect.