To start, I'm a big fan of napkin math, and I agree with your overall conclusion: fiber optics run through a really deep hole in the earth is not an idea worth pursuing. I don't know if you saw it, but 'xoa' had a great example of napkin math a couple weeks ago, explaining why low-earth-orbit satellites were potentially much better than fiber connections for reducing intercontinental latency despite the apparent extra distance: https://news.ycombinator.com/item?id=22185854

> The idea is to see if it's worth doing actual calculations.

Yes, although even more strongly than I worded it in my comment to you, I believe strongly in underlying principle of "first do no harm". In my mind, there's a large difference between using assumptions that simplify the math (like decimal time: 100 seconds in a minute, 100 minutes in an hour, 10 hours in a day), and making misleading factual statements like "Most fiber optic cable has a melting point of 70C".

> That was more the point

I recognize that was your point. My point, perhaps expressed poorly, was that despite your virtuosic example of napkin math, you might be doing more harm than good overall. People will remember not just your conclusion, but your logic as well. If they remember "light travels about 30% slower in a fiber optic cable than in space", this is good. If they remember "you can't use fiber optic cables in a desert environment" that is bad.

Please at least consider this point of view. From your writing, you come off as an expert. Most of what you state is correct, and leading people to trust you. If you then throw in false statements written with apparent high confidence, you will eventually cause people real world harm. It wouldn't hurt your overall argument to stick with the parts that are true, and to make clear that rough estimates are rough estimates.