Mare Island is situated by Vallejo, which is about 23 miles Northeast of San Francisco and at one point was one of California's first capital cities. Mare Island itself has been a significant Navy Shipyard and Naval Base since 1850 and over the decades has grown in size to host an abundance of Military Buildings, factories, hospital's, Master's Quarters, dry docks and even a wonderful chapel with original Tiffany stain glass windows. It was also known for holding/construction of nuclear and building submarines. Sadly in 1996 it was decommissioned and now large parts of it sits empty, left to decay. There are some factories still operating, a few artists have moved in and on one part of the island a golf course has been built and new residential area has been established.
On the southern tip sits the most wonderful Military base I have ever seen, rows upon rows of military quarters, perfectly pristine wooden houses, bunkers, factories and all sadly inaccessible and all barricaded and locked up. I even tried to get to the beach area via a cliff but even that is restricted, due in parts to the warning of possible live ammunition! The northern point of the Island is in even more decay than the South and is filled with Gas Stations, factories and residential flats all sprayed with huge "Not open to Public. Trespassers Will Be Prosecuted" signs on every available surface. Private Security cars constantly patrol and watch all drivers going through the area. They don't take it lightly in the USA.
While it is perfectly fine to walk around the Southern area of Mare Island, I did find that it drew attention and the one time we were stopped it was just a friendly enquiry. It's understandable considering California's homeless situation, and thats is a large part of why everything is Patrolled and barricaded up to the hilt. I am sure there are photographers, "Urban Explorers" that wouldn't blink and would get into large parts of this Island but for me, the Police and remaining Military there scare me too much!
Every week they open up the hiking section (and tease you with views of the base through barbed wired fences), this is run by volunteers keen to keep the history alive. The hike grants you access to the old Naval graveyard, 360 degree views of the bay and an old bunker. It is also possible to contact a caretaker on the Island for tours of some of the Master's Quarters and Chapel. Mare Island probably isn't on most peoples lists of "things to see and do in San Francisco", it certainly isn't on any tour guide I have seen. However it is an important part of the history of The Bay and well worth the visit.
Entry cost to Island: Free
The Preserve (hiking trail): $5 donation
Tour of Masters Quarters/Tiffany Chapel: approx $10-20 donation