Find Sig in Underport
It is also related to a trophy titled Finders Keepers, completing this mission will reward you with a bronze trophy.
The aim of this mission is to reach the end of the area using the Titan Suit. You will be underwater, so you will run out of oxygen with time. Refill your oxygen at air vents located underwater. Watch out for water mines, as they are magnetic and gravitate towards you. There are also squid-like metal heads. You can attack them using the square button, or run past them if you are lucky.