Happy Black Friday! Spectacle Island in Boston Harbor is perhaps the largest expanse of land to build a settlement on. It’s not though, unlocked via the minutemen quests. Instead this goes through the later Railroad Safehouse quest lines.
Note: This is not a definitive play by play walkthrough. However, there is enough information in the linked video and the “Freedom Trail” Video to set you on the correct path to unlocking this amazing settlement. To find the Railroad, follow the “Boston Freedom Trail” in this video here.
Once you unlock the railroad, the Safehouse series of quests, roughly 1-3 hours worth, will lead you to unlocking Specatcle island as both a Safehouse and a Settlement for you to use and build upon. The awesome part is this is not just a section of island, it is the entire island.
The initial downside is this Island paradise is infested with Mirelurks and their queen. Bring guns and friends with guns.
So stop reading and get to unlocking this amazing lot of land and start building your dream settlement.