Before the War a Pulowski Preservation Shelter Executive named Sabbin started constructing secret shelters all over the commonwealth to ensure no matter where he was in the Boston area one would be close by. In all ten shelters was constructed in the following places- Jamaica Plains at the bus stop, Hangman's alley, Red Rocket Truckstop, The Castle by the park , Greentop Nursery near vault 75, Salem near the museum, Boston Airport in front of the parking garage, Forest Grove Marsh near the highway , Glowing Sea near the Outpost, and Starlight Theater near its sign. These shelters come equipped with workbenches and Crates full of Resources and Food, components number in the 10's of thousands each so plenty of building supplies, armor, weapons, food, and other goodies! All the workbenches are linked to the stash box in the interior cell a bed and sink is provided for your survival Immersion, Dogmeat also has a bed of his own. Only the Storage box an workbenches are safe to store your stuff in all other containers are set to respawn for more loot. 

UPDATE: There are now three versions for you to choose from Version 1 is for Settlement Building , Version 2 is for basic almost no loot Player homes to aid in Survival mode , and Version 3 which adds more loot but keeps it reasonable and also adds a unique set of T-45 Power Armor with a chest full of mods for it. The Patriot Power Armor has a Unique Torso which has several of the PA mods built into it, this reinforced suit of T-45 should carry you to higher levels with its reinforced defense ratings. Use only one version at a time.