If you mean on the level 'Cortana' then you must follow the route through the flood space ship and eventually you will get there, it is confusing so it's best to look for a guide on Youtube.
Jun-A266, also known as Noble 3, was assigned to escort Dr. Halsey to ONI's CASTLE BASE on Reach underneath Menachite Mountain. Halsey made it safely to CASTLE BASE and presumably then survived the...