Well how deep a nuclear assault can reach? In Greece there are some witnesses saying that the major internet provider backbone fiber was found buried 10cm below the road. Just after the asphalt inside the pebble.
Also lots of providers just string up cable on poles, which are routinely snagged by trucks, shot by ammunition, run into by cars, etc etc, so even more vulnerable than shallow burial. Some of these cables are major links so damage can cause wide-reaching problems. No match for an out of control sedan, let along a nuclear strike. [4] [5]
[0] https://www.techrepublic.com/article/google-fiber-is-using-a...
[1] https://wpln.org/post/google-fiber-disruptions-have-some-say...
[2] https://www.nashvillescene.com/news/pithinthewind/google-tre...
[3] https://arstechnica.com/information-technology/2019/02/googl...
[4] https://www.wmur.com/article/crash-manchester-comcast-servic...
[5] https://www.cbsnews.com/sacramento/news/crash-vandalism-comc...
and etc
Each hop of the internet uses power. The power infrastructure is above ground for long enough to be overpowered by nukes. So even if the internet were entirely under ground and even if it were entirely only fiber it would need an underground-only power feed coming from an underground-only power generation source. Most internet service providers are above ground. Some telco is underground but only useful for old pots lines and some DS lines. Satellite ground station relays are above ground. Power plants are above ground. Solar panels are above ground.
I could be wrong, so after a nuclear event we should all try updating this thread assuming M5 Computer Security is EMP hardened and has backup power and a fuel contract with a fuel company that still exists. Most data-centers are not EMP hardened.