There are five different elevators in Zero Escape: Nine Hours, Nine Persons, Nine Doors and they allow the characters to travel between decks, provided that they have a keycard for it. The two Mercury elevators are located near the large hospital room and lead to the Bottom Deck, while the two Saturn elevators are located behind the central staircase and lead to E Deck. Both of the elevators allow the players to go below the flooded D Deck because they are water tight. The fifth elevator is located inside the large hospital room and is only used in the Safe/Zero Lost ending by Junpei to return from the bottom deck to C Deck.
The Mercury elevators are located near the large hospital room on C Deck. They are first visited after escaping from the Kitchen or Casino. Junpei jokes that the Mercury symbol on the card reader looks a lot like Lotus, and depending on the route Junpei is on, he will either get his hair violently pulled by Lotus, or tussled by Seven. The key card is found by Junpei (if he visits the Shower room) or by Seven (if Junpei doesn't). Junpei and June are chosen to investigate what the elevator leads to, and find that it takes them to the Bottom Deck and Door 2. If the Jupiter key has been found, they'll realize that all of the numbered doors besides Door 9 have been found, meaning that it will be their next destination. (If the Jupiter key has not been found, they determine that Door 2 must be the way forward). They head back up to C Deck to report the news.
The entire group rides down the elevator to Door 2. There were no arguments on who would go through the door, and Seven, Lotus, and Junpei volunteer (this only occurs if you previously went through Door 3). After some minor flirting with June, Junpei follows Seven and Lotus in through the door. After escaping from the torture room, the group dash down the hallway outside of the exit, ride back up the elevator, and find that no one was waiting for them on C Deck. This made the group uneasy and they started to look around for the other players.
On this route, everybody is excited to get to Door 9 after June's group found it. Clover speaks up and says that they should investigate what lies beyond Door 2, because there might be a hidden exit or a way to get all 7 of them out. Junpei mentions that they are limited on time, but Clover insists that they will be quick about it. They decide that Santa, June, and Seven will investigate the door with Clover. They leave and the elevator rumbles out of sight. Junpei is waiting outside the elevator alone, as Ace had taken Lotus to show her something. A little while later, the elevator opens and only Clover is inside. Junpei asks where the rest of her group is, and she gives him a smug look and throws down their bracelets. She says she killed them because she believes that Santa and Seven had killed Snake and June had tried to protect them. Junpei is in shock and Clover holds her hand out, saying that they can go through Door 9 together. When he is about to get up, Clover slams an axe in his back, takes his bracelet, and taunts him as she is leaving.
Knife Ending/Safe Ending/Coffin Ending/True Ending
On the Knife, Safe, Coffin, and True ending routes, the Mercury elevators are never again used after Junpei and June's initial exploration.
The Saturn elevators are located behind the central staircase on C Deck. They are first visited after Ace, Clover, and Seven decide to investigate the shower room to see if they can find Snake. Lotus suggests that they check out the Earth door and the Saturn elevators, so Santa, Junpei, and June follow her to the central staircase. Santa and Lotus decide to investigate the Earth door while Junpei and June are assigned to the Saturn elevators. The key card was found earlier by Lotus (on the Safe Ending route) or by Junpei (on all other routes). After an innuendo-filled conversation in front of the elevators, Junpei and June ride down to E Deck. They find Door 6 and head back up to meet up with Santa and Lotus.
Junpei, along with Ace, Santa, and June ride down this elevator to get to Door 6 and investigate the rooms behind it. After finding the chapel, they put together that Ace, Santa, and Junpei could go through Door 9 if they wanted to. Instead, however, they agree on going back to meet up with the others so that they won't have leave June behind. They ride back up the elevator and head to the central staircase. There, they find Seven and Lotus. The two tell them that Clover has gone missing somewhere. The group splits up to try and find her. Junpei decides to go with June to investigate. They search the large hospital room, but don't find Clover, so Junpei suggests that they split up. After a little bit of flirting with June, Junpei heads to the Saturn elevator and takes it down to E Deck. When the door opens, he sees Lotus' corpse propped up against the wall. Junpei takes a closer look at her body and finds that her bracelet is missing. As he is about to stand up, Ace stabs him in the back. Junpei collapses on the floor, catching one last glimpse of Ace before he dies.
Like on the Knife Ending route, Junpei, Ace, Santa, and June find Door 9 and go back to the central staircase, only to find that Clover has gone missing. Like before, Clover is not found in the large hospital room. This time, June suggests that they check the shower room, because she might have gone there to check on her brother. Holding his breath to keep out the unbearable stench, Junpei discovers that the corpse is not that of Snake, because the corpse's left arm was not prosthetic. They decide to head back to the large hospital room, but are stopped by Seven along the way. He tells them that Clover is dead and leads them to the 1st class cabin, where everyone else is waiting. Junpei takes a look at her corpse, and starts investigating his surroundings. He eventually finds the safe combination by pressing some buttons on his bracelet and discovers who had killed Clover, "Snake", and the 9th Man. Junpei asks everyone to come with him to the large hospital room, and they all follow him there. Once there, Junpei and Ace have a heated exchange, which ends with Junpei managing to trick Ace into revealing that he had killed everyone. Ace is proud of what he has done, so an angered Lotus drives her fist into his nose. Ace mocks her and manages to grab her from behind, pushing a revolver against her head. He leaves the room and drags Lotus to the elevator, taking her to the small 9 door. Moments later, Junpei and Seven arrive at the chapel, finding them already gone.
The elevator is also used later, after Junpei and Seven had found Snake and taken him to the incinerator. After they are forced to leave Snake to burn with Ace, Junpei wearily tells Seven and Lotus that he is going to go retrieve Santa and June. He takes the elevator near the incinerator back up to the large hospital room on C Deck, and looks for them in the large hospital room. To his surprise, they aren't in there. He decides to take the Saturn elevator back down to the chapel and finds June there, lying on the floor. She confesses her love for him. Shortly after, Zero's voice crackles on the loudspeaker. Zero tells Junpei that he can't escape, because he "chose the wrong path". The door behind Junpei slams shut, and he leaves June for a moment to investigate. He finds nothing, and goes back into the chapel, only to find that June has disappeared. A Soporil grenade is then thrown into the room, and Junpei collapses.
Axe Ending/Coffin Ending/True Ending
On these routes, Santa, June, Seven, and Lotus investigate what is behind Door 6, and thus, have to take the elevator down to E Deck. They eventually find the chapel, and find that everyone except June can escape through Door 9, but Santa opposes the idea. The group heads back to the central staircase to meet up with Ace, Clover, and Junpei, who had just recently escaped from the Captain's quarters. Upon revealing the news about finding Door 9, the group is excited and either decide to investigate Door 2 first (on the Axe Ending route) or head directly to Door 9 (on the Coffin Ending and True Ending route). The seven of them cram into the elevator and are restless on the ride down. They get out and head towards the chapel, with no idea of how all seven of them are going to go through Door 9.
Safe/Zero Lost Elevator
This elevator is only shown in the Safe/Zero Lost ending, where Junpei uses it to return from the Bottom Deck to the large hospital room on C Deck to look for Santa and June. It is neither mentioned nor used at any other time during the game. It does not require a keycard to activate, but rather must first be activated from the Bottom Deck. It is unknown whether it remains usable after initial activation, but based on the behavior of other similar elevators, it is a distinct possibility.
Strangely, despite visiting the room several times over the course of the game, the characters never mention its presence nor is any image of it shown outside of the ending in which it is featured.