Infiltration

 is a main quest in Fallout 3 which was cut from the final game. Its quest stages can still be found in the game files but beyond that there appears to be nothing left of it.

In this quest, the player was supposed to enter Rivet City through an underwater hatch, acquire the energy source of "the prototype" and return it to Scribe Rothchild in the Citadel. The final objective was to meet Elder Lyons in his study.

The quest's editor ID is "MQ10". The prefix MQ designates it as a main quest and the number places it between The American Dream (MQ09) and Take it Back! (MQ11).

Interpretation
The following is based on the author's interpretation of aforementioned facts and is not necessarily true.

The "prototype" is possibly what Dr. Li was working on before rejoining James at Project Purity. Its energy source would probably be needed to reactivate Liberty Prime, based on the fact that its energy problems are described in numerous terminals at the Citadel. In the final game, it is simply mentioned that Rothchild and Dr. Li managed to solve its energy problems but it is not specified how; this quest was probably supposed to fill this gap.