To avoid any confusion I can confirm the random sites are the sewer/dungeon/crypt located in the well next to the Site Foreman (ladder or shovel symbol on the map).
The ladder indicates that you've unlocked that specific site where the shovel indicates you still need to unlock it, to unlock a locked site just head there, clear the enemies and upgrade the guard station, once you've done this head over to the shovel symbol shown on the minimap and you'll have an option to activate the site, activate it with reclaimed resources and you'll be able to access that particular site.
Now to get the achievement you'll have to explore/finish 50 of these, I would recommend grabbing some Alliance members, it's not entirely necessary but if you're lacking in DPS it never hurts.
YOU CAN TRACK YOUR PROGRESS BY GOING JOURNAL/ACHIEVEMENTS/EXPLORATION IT'S LOCATED AT THE BOTTOM
Hope this helped!