The Mariner was a Zion hovercraft that was crippled by a sentinel attack while its entire crew, save the operator, was inside the Matrix. Rook, the operator, managed to destroy the sentinels using his lightning rifle. But not before the almost the entire crew got killed, including himself. The only survivor, Rocket, had his leg broken during the attack and, with no one left to safely plug him out, was left trapped inside the Matrix.
Five days after the attack, the Mariner was successfully located by another hovercraft, the Pequod, using the information given by Rocket. The Pequod had even established an exit and were already calling Rocket in a hardline when, unfortunately, Rocket was killed by Agent Brown before being able to answer the phone.