Rock Creek Caverns

The Rock Creek Caverns are located in the east/southeast portion of the Capital Wasteland. It is located northwest of the National Guard Depot. There is a random encounter where three adventurers are attacked by a Deathclaw. If you save them, one of them will give you the location to Rock Creek Caverns. Even if you just run from the (heavily injured) death claw instead of helping them you will get the map if the main adventurer is still alive to talk to.

The Caverns are accessed through a sewer grate at the bottom of a radioactive-water filled crater that has exposed the sewer system. The sewer network is attached to a short segment of the Metro system, which then descends into a cavern system filled with Mirelurk, Mirelurk Hunter, and Mirelurk King creatures. There is lots of water - a Radiation Suit or Advanced Radiation Suit will come in handy.

Notable Loot
At the end of the cavern system, there are a number of dead adventurers and an ammunitions cache, with (among other things) a Laser Rifle, Laser Pistol, Power Fist and associated ammunition. Also near the weapons cache is an outcrop of scaleable rocks which will lead up to a Nuka Cola Quantum. The actual treasure is most likely the valuable Mirelurk Meat (valued at 20 caps ea.) and Softshell Mirelurk Meat (valued at 30 caps ea.), which can often be found in the eggs sacks littering the floor of the tunnels.

Actually, there's at least 2 safes/ammo caches, second one is when you keep right (eastward) after the long tunnel. Less swimming that way, too.

In an alcove there is a briefcase containing possible clothing, such as Sexy Sleepwear or business attire.

this should help you have a good loot and go home with loads of caps