There was still one unsolved mystery in the serial locked room murder case in Yatsuwako Village: How was Tabishirou Monokaki, the victim of the Locked Storehouse, brought back to the village? Because while the village's entrance was unlocked, there was always a gatekeeper keeping watch. Previously, Mitsumura had said that Tabishirou was brought in through the underground limestone cavern, but that's impossible. At the time, the underground limestone cavern was a vacuum, and when the door was opened, air would have gone rushing in. The speed at which air flows from a space of one atmosphere into a vacuum is 340 meters per second. If the culprit had tried to bring Tabishirou in through the underground cavern, the moment she opened the door, wind would have immediately blown her into the limestone cavern at 340 meters per second. Even if she was able to survive that, when she opened the forbidden door to the mansion's backroom, she would have been hit by another 340 meter per second wind. That meant it was absolutely impossible for the culprit to have carried Tabishirou in via the underground cavern.
Which left the only route the culprit could have taken as the tunnel, which served as the village's only entrance. However, that tunnel was under constant surveillance by the gatekeeper, making it another locked room. In other words, it was the tenth locked room of Yatsuwako Village.
So, how had the culprit gotten into that locked room? Mitsumura had already deduced the solution, and evidence of the trick was found by the police during their investigation.
The day we were released from Yatsuwako Village, Mitsumura and I had the following conversation on the train ride home.
“The tenth locked room, the Locked Tunnel, isn't very difficult. You see, the ceiling, walls, and ground of that tunnel were all covered in brick, correct? For a hundred meters from the entrance. So the gatekeeper was inside an enormous cylinder of bricks. So the culprit, Miss Teika, removed the cylinder. Slowly, so that the gatekeeper didn't notice. And once the cylinder was removed, the tunnel was deserted. And so, Miss Teika could place Tabishirou in her trunk and casually walk right through.”
It was a crazy trick, but after seeing all the other large-scale tricks in the case, it felt plausible. There was probably a mechanism to slowly move the brick cylinder automatically in response to some signal. The tunnel was about 130 meters long, and was brick for the first hundred meters. The wire mesh blocking the tunnel was on the boundary between the brick cylinder and the sandy area beyond. So after the mesh lowered and the village became a closed circle, it became impossible to enter, even with the cylinder out of the way. But when Tabishirou was brought in, the mesh wasn't lowered, so there were no problems there.
The gatekeeper was on guard in the center of the brick cylinder, so the distance to each end of the cylinder was 50 meters. Moreover, the lighting inside the cylinder was dim, so if the cylinder was moved in the middle of the night, it would have been extremely difficult to notice the change in the scenery visible outside the ends of the cylinder. The gatekeepers' shifts were 3:00 A.M. to 7:00 P.M. So Teika must have moved the cylinder after 3:00 A.M., and returned it to its original location before dawn. In that case, if Teika had summoned Tabishirou somewhere near the village beforehand, she could have left the village, knocked him unconscious, and brought him back into the village, all without being seen by the gatekeeper.
Of course, Zerohiko Monokaki was the one who'd come up with this scheme, and as it was a trick not mentioned in any of the locked room drafts, it was likely his own creation. He then lied to the villagers about “renovating” the tunnel, and spent many years and a massive amount of money to complete it.
“I am in awe of his passion,” said Mitsumura, gazing out the window to the view scrolling by. “In the end, a billionaire who creates their own locked rooms may be the ideal culprit.”