What does the number 18 mean in Silo? Explained

There are only a couple of days before the second season of Silo wraps up, with the season finale dropping on Apple TV+ this Friday, January 17, 2025. However, there are still a few secrets and mysteries that are yet to be revealed about the dystopian sci-fi drama. For one, the number 18's meaning after it came up again during last week's episode.

At the near end of episode 9 of the second season, Robert Sims mentioned knowing that Bernard has a keychain with the number 18 etched on it, bringing back the intrigue about what the number signifies. Number 18 in the show refers to Silo 18, the identification of the 18th of the 50, possibly 51, underground bunker complexes that house around 10,000 people, of which Bernard Holland was in charge.

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The meaning of the number "18" in Silo explained

The number 18 in the series, the label on Bernard Holland's keychain, and the hard drive from Juliette Nichols' boyfriend, George, were an identification mark of the underground bunker complex in which Bernard was in charge. As he confirmed in the penultimate episode of the second season, there are 51 Silos in total, with one most likely serving as the headquarters of the founders.

The number 18 refers to the 18th of the 50 Silos that house tens of thousands of residents, the one where Bernard became Mayor and where Juliette Nichols was originally from. Bernard's keychain says the number 18 because it's the key used to open the server room on the 18th, and the hard drive says "18" because it stores data from the 18th.

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The times when the number 18 appeared or was mentioned in the series

Throughout Silo season 1, the number 18 has appeared or is mentioned multiple times, especially during the last two episodes. In episode 9, The Getaway, Robert Sims went to Bernard's office after Juliette Nichols escaped. As they discussed their current predicament with Juliette, who has a red-level relic, Bernard's keychain on his desk started flickering with a red light. It was an ordinary enough keychain, but the number 18 was etched on it.

Also, in episode 9, Bernard interrogated Lukas Kyle, asking what Juliette wanted from him. He confessed to seeing the relic, aka George's hard drive. Bernard insisted that he describe it, further demanding him to tell the hard drive's serial number. Lukas said there was no serial number on the hard drive but number 18 was written on it.

George's hard drive with the number 18 (Image via Apple TV+)

Bernard's number 18 keychain appeared again when he used it to open the server room, as seen in the Silo season 1 finale episode. It was flickering red again when Bernard rushed to the server room after Juliette crossed the hill, and he said, "She knows," referring to Juliette finding out that the image on the visor was fake.

In season 2 episode 9 of the series, Sims brought up Bernard's keychain again during his conversation with his wife, Camille. They were talking about siding with the people from down deep, helping Paul Billings, and, possibly, Sims becoming the leader after the rebellion, but he said he couldn't do it because the people saw him as the enemy. However, he said that there was something else he could do, adding:

"Bernard has a key. Number 18 on it. And when things go sideways, it lights up and buzzes and he disappears."

He confirmed that it was the key to the server room but that he didn't know what was inside it.

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Catch the complete installment of Silo season 1 only on Apple TV+. The previous nine episodes of season 1 can also be watched on the platform, with the season 2 finale coming on January 17, 2025.