I love Numberphile, and I love this kind of Numberphile specifically: here, starting from small numbers using ideas and operations that are extremely easy to understand and (at some scale) iterate by hand, is a wild unsolved problem absolutely beyond the reach of our current mathematical or computational capacities:

https://m.youtube.com/watch?v=OtYKDzXwDEE&pp=ugUEEgJlbg%3D%3D

An amazing thing about 276 - Numberphile

YouTube

I think the thing I love most about this is that is, like so much involving prime numbers and factorization, an extremely powerful brand of catnip for amateur crank mathematicians such as myself. You can _kind of see_ the patterns there! There are the barest hints of a _lot_ of subtle structure. The secret must be within our grasp!

I mean, does this not look _just like_ the 3x+1 problem, turned on its head? Almost? Maybe?

And yet.... and yet....

C. S. Lewis once described The Occult as a series of nested snares, symbols for symbols for symbols without ultimate referent, a machine purpose-built to be an intellectual rat hole, seducing and confusing the inquisitive mind, snaring the most inquisitive of minds that much more effectively.

But, in a very Lovecraftian way, there are things that we can find right here in nature fully capable of that, of having the shape of the occult - or rather, a true shape, aped badly by the occult.

@mhoye don't follow the objects best not named unless one wants people to follow them out into the depths lest you turn into an algebraic number theorist, for if you do, you will never forget the horrors that lurk behind the number two.
@feonixrift @mhoye and monsters hiding in the numbers is Charlie Stross' _Laundry Files_ books.