Welcome to Arcadia, Time to Die
IGN was able to wander to and fro and even needed to access a map to find our way around the lush labyrinth of Arcadia. Backtracking was pretty common and there were several routes around the very green area. While the map in this level wasn't Oblivion huge or anything, there were definitely more than a few areas to traverse to find all of the necessary components to piece together the Lazarus Vector.
Upon arrival to Arcadia, the felonious Andrew Ryan is there to greet you with his dastardly plot to suffocate you by destroying the city's oxygen supply. Arcadia just happens to hold the lion's share of the oxygen producing trees needed for Rapture to survive as a whole. Seeing his opportunity to destroy you while potentially destroying plenty of other inhabitants as well he begins gassing the plants with a deadly mist. Your only ally Atlas quickly spouts the location of a botanist in the zone that can reverse the effects using a substance known as the Lazarus Vector.