Silo seasons 3 and 4 have already been confirmed, and there are plenty of massive questions waiting for answers when Apple TV+’s hit sci-fi series returns.

Based on Hugh Howey’s series of novels, the show has always been built on big mysteries about the origins of the titular underground Silo that 10,000 people call home. But as Sheriff Juliette Nichols (Mission: Impossible’s Rebecca Ferguson) and her allies have discovered more about their vertical metropolis, sinister forces (led by Silo mayor Bernard Holland, played by The Shawshank Redemption’s Tim Robbins) have closed ranks to protect its secrets.

Season 2 storylines have involved Juliette being trapped in one of the many other Silos, her former colleagues in Mechanical rebelling against the powers-that-be, and plenty of revelations to make you query everything you thought you knew about Howey’s ingeniously realised world. Of course, as with Apple TV+ stablemate Severance, every answer the show has delivered has raised another pertinent question – so here are 10 burning questions Silo season 3 needs to answer. We can’t wait…

***Warning: spoilers from Silo season 2 ahead***

1. Have Juliette and Bernard survived?

At the end of season finale “Into the Fire”, Juliette Nichols (Rebecca Ferguson) finally makes it back home to Silo 18. Before she can make it through the airlock, however, embittered mayor and head of IT Bernard Holland (Tim Robbins) turns up brandishing a gun. Their tense conversation is subsequently interrupted when the Silo’s sterilisation procedure activates, engulfing the corridor in flame. Will they make it out alive?

It’s pretty much guaranteed that Juliette will return – she’s the main character in the show, Ferguson is an executive producer, and showrunner Graham Yost alluded to his plans for the character in season 3 in an interview with The Wrap.

Bernard’s fate is a little more up in the air. The character dies at the end of Wool (Hugh Howey’s first Silo novel and the inspiration for the first two seasons), but the show has been known to play fast and loose with the source material before. In other words, all bets are off.

2. What did Lukas tell Bernard?

We may never know what Bernard’s IT “Shadow” Lukas Kyle (Avi Nash) whispered to his boss on the stairwell, but it appears he divulged the existence of the top-secret “Safeguard”, a euphemism for a mechanism built into the Silo that can release poison gas and kill everyone inside. Learning that “they can kill us any time they want” prompts Bernard to question his life’s work as guardian of the Silo’s mysteries, before setting off for a walk “outside” so he can “feel free for one f***ing moment of my life”.

3. Who created the Algorithm?

It turns out Bernard was never actually in charge of the Silo. In episode 9, “The Safeguard”, Lukas’s investigations into the code hidden in the late Salvador Quinn’s copy of the Pact lead him to the bottom of the Silo – only Quinn, Judge Mary Meadows and George Wilkins, Juliette’s late boyfriend, had ever been there before him.

In the basement he encounters a creepy digital voice that already knows his name and threatens to initiate “the Safeguard” if Lukas tells anyone about their meeting. Subtitles for the episode reveal that this entity is known as “the Algorithm”.

The Algorithm has all the hallmarks of sophisticated AI, but if that’s the case, who created it? Alternatively, it could be a human (or group of humans) masquerading as a computer – rather like the Wizard of Oz – but if so, where did they come from and what are their motives?

4. What’s the wider purpose of the Safeguard?

We know that it’s controlled by the Algorithm and capable of wiping out an entire Silo – we suspect the Safeguard is what killed the residents of Silo 17 when they escaped to the surface.

But its very existence puts a sinister spin on the Silo’s existence. If, as we’d always assumed, the Silo was designed to protect the people inside from certain death on the surface, why build a Safeguard capable of killing every single one of them? Why is the Algorithm so concerned about its secrets getting out into the open? And what could be so important that 10,000 people are considered expendable to protect it?

Juliette tells Bernard she’s “figured something out” to stop the Safeguard being instigated, but will that be enough?

5. Why does the Algorithm want to speak with Camille?

“Why are you here?” asks the Algorithm when Judge Robert Sims (Common) finally reaches the Vault.

“I want to save the Silo,” he replies, before the Algorithm responds: “So do I.”

It then orders Sims and his son, Anthony, to leave the Vault, but tells wife Camille (Alexandria Riley) she can stay.

But why has the Algorithm earmarked Camille for special treatment? She’s generally the smartest person in the room, consistently coming up with strategies to benefit both her family and (on occasion) the Silo. It’s reasonable to assume the ever-present Algorithm has been watching her and spotted her potential.

6. Will the Silo continue in its current form?

Even if Bernard survives, he seems done with the concept of the Silo. That means new management is inevitable, but who will be asked to lead in his place? Camille and Juliette seem like prime candidates, but will either want the job?

Besides, after spending the best part of season 2 rebelling, Mechanical will surely have a part to play in Silo politics – but not before they’ve rebuilt the stairways they blew up to trap the Raiders (the military wing of Judicial) down below.

7. What's the Washington, DC flashback all about?

With Juliette and Bernard about to be engulfed by that fireball, Silo takes its biggest swing yet by shifting the story to a more contemporary Washington, DC. But this isn’t quite our present, as we discover when a security guard scans Congressman Daniel Keene (Ashley Zuckerman) for radiation.

Daniel thinks he’s meeting Washington Post journalist Helen (Jessica Henwick) for a date, but she’s more interested in talking about a recent dirty bomb attack on New Orleans. She wants to find out whether the US has any plans to strike back against suspected aggressors Iran, and whether or not this radiological attack actually took place.

Set well over 300 years before the rest of the show (Bernard has previously confirmed the Silo is 352 years old) this prequel angle is likely to be a major component of the Silo season 3 story. It will presumably answer questions about what happened in the outside world before the Silo was built. Maybe we’ll also learn how the duck Pez dispenser Daniel gifts to Helen turns up as a forbidden relic in the Silo centuries later?

8. What about the other Silos?

Juliette hails from Silo 18 and spent the whole of season 2 in Silo 17 before making a triumphant return. But with another 48 or 49 (depending on your source) Silos out there, there must be other stories to tell.

Will Juliette be reunited with Solo (Steve Zahn), who’d survived alone in Silo 17 for decades? Have any other Silos rebelled against the Pact? Has the Algorithm initiated the Safeguard elsewhere? Is the Algorithm overseeing every Silo at once, or is there an individual Algorithm for each Silo?

However this plays out, the story must surely stretch beyond the walls of Silo 18.

9. Is the world outside the Silo genuinely unsafe?

Prior to Juliette’s exile, everyone who left the Silo to “clean” was dead within three minutes. That doesn’t necessarily mean the Earth’s surface is permanently toxic, however.

Describing the day his fellow Silo 17 residents died, Solo tells Juliette: “Then that dust started to blow again and I think the poison went away for a bit but it came back and a lot of it, and that’s when they all died.”

Could it be that this “dust” was actually associated with the poison gas unleashed by the Safeguard, and that similar measures were used to kill everybody sent out to clean? (Although previous exiles died, Juliette’s suit, with its superior sealing tape, could have protected her from the gas.)

And if you wanted to ensure thousands of people remained compliant in a giant pit, what better way to do it than telling them going outside will kill them? In the aforementioned flashback scene, Helen isn’t even sure the nuclear attack ever happened. Could the Silo have been built on a lie?

10. Why were the Silos built? And by whom?

This is the fundamental question of the show, so don’t expect definitive answers until Silo’s fourth and final season. Bernard tells Juliette that he “knows the who”, but even he’s in the dark when it comes to the why.

Silo seasons 1 and 2 are now available on Apple TV+. Silo season 3 is currently in production and a release date is TBC.