Iron Lung is an achievement/trophy featured in Call of Duty: Black Ops III. It requires the player to stay underwater for a total of one minute.
In order to complete this, the player must go to the underwater tunnels by Mule Kick and go underwater. Within the tunnels are spores which when shot will spurt out air bubbles. The player must go up to the spore when it explodes, which will refresh the player's air. Doing this for a minute will unlock the achievement/trophy. Alternatively, the player can use a Gas Mask to increase the durability of the player's breath underwater.