The entrance to Rock Creek Caverns is a sewer grate at the bottom of a crater that has exposed the sewer system, just to the north-northwest of the National Guard Depot. When coming from the direction of the National Guard Depot, there is a guaranteed encounter with two raiders and two super mutants. Some areas are moderately radioactive.
This section is the only accessible area in Rock Creek Caverns. It has a large central chamber numerous tunnels and offshoots including areas of the metro system. 15 mirelurks are scattered around the cavern. At point D on the local map image seen above, there is a bed that can be slept in along with a safe. At Point F there is a mercenary and a wastelander corpse or two, along with an Easy-locked safe which may contain a Stealth Boy, a power fist and a few ammunition boxes. Point G has a dead skeletal family and a stimpak and RadAway, along with a teddy bear. Point E just has some dead brahmin and dead ants along with some egg clutches and Point C has a car near the ceiling, (apparently where the road once passed through), which will explode if shot at.
A Nuka-Cola Quantum can be found in a wooden crate at B on the local map image. To get to it, walk up the fallen stalactite at A, which rests at an angle against a wall in the first and largest chamber of Rock Creek Caverns. At its tip, jump up to where some mushrooms are located. Hugging the wall, walk on the skinny ledge around to B.
An oversized glass pitcher can be found in the same area as the safe.
An oversized cup can be found in the same area as the safe.
There is a random encounter where three adventurers are attacked by a deathclaw. If you save them, one of them will give you the wastelander map, which shows you the location to Rock Creek Caverns. Even if you just run from the (heavily injured) deathclaw instead of helping them, you can still get the map if the main adventurer lives to talk. Sometimes if you wait a while or fast travel back to the location the wastelanders will reanimate, if they were killed, and you will be able to talk to them. For more info, see Fallout 3 random encounters.
When you first enter the Rock Creek Caverns, there are a couple of gates in the sewers that bar your advance. Turning off clipping shows that there is nothing past these gates, or any special ways to get past them.
Behind the building, directly to the south of the entrance to Rock Creek Caverns there is a scripted encounter between two raiders, both carrying missile launchers and two leveled super mutants carrying either miniguns at lower levels, or two Gatling lasers at higher levels. At level 20 they are super mutant masters, and at level 30 super mutant overlords.
Upon reaching the isolated metro section at G, closer inspection reveals a knife and whiskey bottle on the ground near the skeletons and baby carriage.