The butterfly effect. The phenomeon that tiny seemingly insignificant changes can result in massively different outcomes. Someone out there could read this post and get distracted and leave home for work/school/shopping a bit later than they would've and avoid a major accident. But conversely, someone could also get distracted by this post while crossing the road and... you know... die...

Fascinating, yet terrifying at the same time.

The scale of the universe. It's an incomprehensible amount of emptiness.

Galaxies are not evenly distributed in space. Instead, when you look at the universe, galaxies are grouped in giant strings that look like a neural connections in a brain.

The concept of emergence blows my mind.

We have this property in our universe where simple things with simple rules can create infinitely complex things and behaviours. A molecule of water can’t be wet, but water can. A single ant can’t really do anything by himself, but a colony with simple pheromone exchange mechanisms can assign jobs, regulate population, create huge anthills with vents, specialty rooms and highways.

Nothing within a cell is "alive", it’s just atoms and molecules, but the cell itself is. One cell cannot experience things, think, love, have hopes and dreams, or want to watch Netflix all day, but a human can.

The fact that lots of tiny useless things governed by really simple rules can create this complexity in this world is breathtakingly beautiful.

Kinda ties into your example :)

I am focusing on the "blow my mind" part, rather than the "beautiful" part of your question, but I am certain many philosophically-minded people would consider the following "beautiful".

Peter Singer's argument in "Famine, Affluence, and Morality (1972)" that you and most everyone you know are probably immoral or evil and you don't even realize it. It really affected my ideas of how to strive to live.

Here is a good video explaining the idea in detail, worth 30m of your time.

Peter Singer - ordinary people are evil

It would have to be Gödel's Incompleteness Theorem. Such a beautiful proof that shakes mathematics to its core.

The science communicator Veritasium made a nice video about it: https://youtu.be/HeQX2HjkcNo

I first learned about it in Douglas Hofstaedter's masterpiece Gödel Escher Bach: An Eternal Golden Braid

BitTorrent. I only need to share a file once and it could potentially reach millions of people. It's old tech now but it feels like magic to me.

Noether's Theorem:

https://en.wikipedia.org/wiki/Noether%27s_theorem

https://math.ucr.edu/home/baez/noether.html

Fundamentally, it allows us to logically infer the conservation laws from the laws of motion of a given physical system using relatively simple math. It always applies, no matter if we're talking about massive systems or quantum ones.