Like, in Boston where there's underground tunnels? There are none.
However, there are caves that lead to underground corridors where at the end there's a treasure, usually a trinket.
You find these by looking around. A good idea is to mark the chest that's underground on your map, and then explore the general vicinity of the chest until you find the cave. --- "...Pretty much boosting your boosting"-- DeliciousJello Official Daniel Cross of the ACIII boards
If you mean Fast Travel Points there are not much: the one in Connor's village (you'll have that one from the beginning), one in Lexington (They are Carriages near the general stores), Concord and Monmouth
In theory you just need to walk over them. For me only one was unlocked, the other one showed up periodically and Monmouth not at all. But you can Quick Travel to the Forts and habors --- It's so much easier to see the world in black and white. Gray? I don't know what to do with gray. Garrus Vakarian
The Monmouth fast travel point appeared for me after the most recent patch.
Maybe the OP was asking about the entrances to the various caves needed to access some of the trinkets and chests? --- We're having a great time: food, wine, a little personal abuse, and the universe going foom.
Climb on the stores to reveal the map, that's how i get them to show up. --- We work in the dark to serve the light. We are Assassins. Nothing is true, everything is permitted. Official Yusuf Tazim of the AC3 boards
that said how do i find other entrances in boston? as best as i can tell ive gotten 4 on the east side, and then theres a door locked with a bar on the opposite side and it seems the only way to get there is to enter from another entrance to the east --- Proud LUEser i'm playing the game and suddenly, outof no where, the germans own the jews :/-Warrior Hart
It's not always a hole sometimes you have to use a lift or walk through water or climb the wall. If you have trouble you can always go on YouTube :-) --- It's so much easier to see the world in black and white. Gray? I don't know what to do with gray. Garrus Vakarian