I’m reading Wool for a book club I’m in and we are all very confused. I’m on chapter 12 and they’ve just made mention of the fact that it takes them two days to descend the stairs of the silo and will take them 4 to go back up.

None of us understand how. Even if these “floors” were double sized, or even triple sized, and the people doing the walk are old, this should take at most a few hours at an absolutely glacial pace.

  • Judging from the schematics in Valorum's answer, the stairs are very narrow. The traffic on the stairs may hold travelers up for a while. (As you read on in the series, some background to this comment may become evident.) Also, IIRC the mayor takes the opportunity to talk to a number of people as they pass, since she rarely gets to meet the constituency. Dec 24, 2022 at 6:25

2 Answers 2


In the sequel novel, Shift, the building of the silos is described. One of the architects states:

‘Do you know how many floors deep this thing is gonna be? If you set it on the ground, it’d be the tallest building in the world.’

Assuming this isn't hyperbole, and also assuming that there was no increase in the world's tallest building between publication in 2013 and 2049 when the construction was set, this would be the Burj Khalifa, 828m (and 162 floors) tall.

Average speed of descent is around 0.4-0.8m/s (based on much smaller buildings), which would indicate about 15-30 minutes to descend that distance.

Either buildings got a lot taller in the intervening years, people became much less active, or it remains a bit of a plot hole.

  • 1
    "Assuming that there was no increase in the world's tallest building between publication in 2013 and 2049" - You're joking, presumably. Just in the last 20 years the world's tallest building height has nearly doubled and there are serious plans for buildings that are 3-4 times higher than that; processindustryforum.com/hot-topics/…
    – Valorum
    Dec 23, 2022 at 20:00
  • 1
    Up to a point. Using available data. However, 20 years doubling would still take us from minutes to hours in 36 years, not multiple days.
    – Michael
    Dec 23, 2022 at 20:12
  • Well, based on the rate of growth, by 2049 we could reasonable expect the tallest building to be in the tens of KMs high :-)
    – Valorum
    Dec 23, 2022 at 20:46
  • 3
    I may not be a civil engineer, but as an engineer, I'm going to weigh in here and say that assuming linear (or exponential) growth will continue in perpetuity is a very bad bet. Examples: Aircraft speeds, Moore's law, F1 - pick anything really. Whatever you're designing, you usually run up against some physical limit. In the case of buildings, you'd have to ask a material's scientist, but I'd bet a lot of money you'll run into some practical problems that's going to stop things from doubling in size a very long time before you get to 10s of kilometers. Dec 23, 2022 at 23:15
  • Ya, that was one Google search to find someone who starts spelling out exactly all the problems you would have qr.ae/prhylq 😂 Dec 23, 2022 at 23:20

Hugh Hovey hosted a series of fan-made but author-endorsed schematics. From this we can deduce that the staircase isn't spiralling around the outside, but is rather more central. The cross-section has each floor being around 200+ feet wide, but there's no indication of height (or rather depth).

We do know, based on the fact that several of the "floors" have hidden bunkers and/or are able to host large equipment like power generators, that they're not just single-sized floors (like you might find in a skyscraper) but possibly each landing could well be leading up and down to sub-floors, or just have very high ceilings. That would explain why crossing a set of 30-40 floors can take a whole day if they're having to head down the equivalent of 5-10 floors per 'level'.

‘Twenty floors in just over two hours. Don’t recommend the pace, but I’m glad we’re this far already.’ He wiped his moustache and reached around to try and slip the canteen back into his pack.


Assuming a sedate walking pace, gives us a rate of about one level every six minutes. That would imply that each floor has a depth of about 80-100 feet, and the Silo a total depth of 12-15km, not accounting for the mines under it.

This is backed up in the sequel novel Dust. What we would consider to be a normal 'level' (roughly the height of a building floor) is considered by our heroes to be less than half or a quarter the height of a level in Silo 17.

They had explored the bunker down to the bottom – it was only twenty levels deep, and the levels were so crazily packed together that it was more like seven levels tall.

This also doesn't taken in account out-sized levels like those found in the Engineering and Mechanical districts.

  • 2
    Deepest mine now is 4km - and there were 3.5km examples in the 50s. Would imply a big advance in technology - not least the cranes used to drop complete sections down a 15km shaft.
    – Michael
    Dec 24, 2022 at 12:27
  • @Michael - this book is set in the future. The makers of the silos also had decades to prepare. How deep do you reckon you could go with the entire US economy behind you?
    – Valorum
    Dec 24, 2022 at 12:49
  • 1
    Construction was in the future, but decades not centuries. Could do a lot, but limited if it was relatively secret - especially if looking at revolutionary tech rather than progression of existing techniques.
    – Michael
    Dec 24, 2022 at 23:14

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