I have no technical understanding of fiber networks. The explanation that this was caused by one fiber line in Brooklyn being severed seems unlikely. Curious to see what the experts think.
Then they get to dig it out better and someone gets to get down in the hole and figure out which piece of fiber needs to be spliced to which other one.
Because of DWDM, some of those individual fibers may each be carrying 64 channels that in turn each carry 400gbit/sec for different providers/services/etc.
It is still a lot of work, and so far not automated.
You should take some time to understand the physical layer of infrastructure. There will always be work to do in meet-me-rooms!
Source: I have no idea.
So imagine if you cut a Cat6 cable in half, and then spliced the individual wires back together without regard to which wires you connected. Very likely, Pair 1 negative is connected Pair 3 negative, Pair 2 has its polarity reversed, etc, and so the network link never comes online.
At the individual device level, fiber cables are connected in pairs, in a crossover fashion. Device A Tx connects to Device B Rx, and vice versa. So with a random splicing, you would end up with pairs getting crossed or broken up, invalidating the connections and labeling of every patch panel downstream, making for yet more work than the initial work of splicing the cut bundle.
1. Not impugning your intelligence, knowledge, or character here, some people never have because they've never needed or wanted to, and that's totally okay.
The major internet connections are like superhighways, with major central routers acting like exchanges. If a major superhighway had an accident blocking traffic, there would be personnel stationed at exchanges to help re-route the traffic, though that often occurs too late to prevent a bunch of backups. At any rate, people using Google Maps, Waze, etc. would see the accident and would likely be redirected to alternate routes that would normally be much slower - and would also be slowed down by the influx of higher than normal traffic.
The fact that the internet still worked albeit much slower than usual leads me to believe that something similar happened. While normally my internet traffic might go through a major hub in NYC and then onward to various carriers and hosts, etc. it had to be re-routed around that hub that is normally sub-optimal, and also ill-equipped to handle the spillover. (And in some cases, bandwidth was saturated, and failed altogether for some users for certain usage.)
Note: I am not a fiber technician nor do I work for a large ISP.
Too much traffic is concentrated at the major carriers which carry a large amount of traffic over relatively little fiber (cause its cheaper) and when that fiber disappears we lose a massive amount of capacity...