Once you reach the 33rd mission "Down Below" theres no going back to freely explore, so take your time before starting it. You'll get warnings from characters and on screen when youre getting close, definitely pay attention to those since theyre easy to miss during intense moments. Something interesting about this point, you can still do quite a bit if you make a backup save file right before heading in. Good idea to wrap up any remaining side jobs, gear collection, and artifact hunting cause once you start that mission, everything else auto-fails and you're locked into the story path. The warning shows up during "The Road to the Foundation" which gives you time to prepare, but its smart to handle any unfinished business in the Zone before pushing forward. Most folks stack up on ammo and medical supplies here, since the final stretch can get pretty intense and theres no backtracking for forgotten equipment.