Door 8 is one of the second set of numbered doors the group encounters. It is located on C Deck, inside the large hospital room. It leads the players to the Laboratory. It is the door on the far right side of the room.
After an unsuccessful search for Snake, everyone regroups and decides to move forward. Lotus proposes to leave one member behind, due to incompatibility of digital roots. It would be it impossible for all 7 of them to pass through any of the doors. Ace volunteers to stay behind, injecting himself with Soporil so that everyone is forced to leave him behind despite their objections. It is later revealed that Ace had stayed behind so that the group could not go through Door 3 and see the corpse of "Snake".
That decided, the group of 6 are left to choose which door they wished to enter. Junpei decides to go through Door 8.
With Ace out of the picture, the digital root of the remaining group is 3+4+5+6+7+8 = 33 = 6. This means that the only way for them all to go through is if they take Doors 7 and 8 (7+8 = 15 = 6), making Door 3 unavailable unless they were prepared to leave even more people behind.
Behind Door 8
Upon entering, the DEAD is near the door and easily found. The hallway the party found finds in was long and winding, but the only point of interest is the large iron door at the end, so the walk there is short despite the hallway's length. The three enter the door and find themselves in the Laboratory.