The p-adic numbers are bizarre alternative number systems that are extremely useful in number theory. They arise by changing our notion of what it means for a number to be large. As a real number, 1 billion is huge. But as a 10-adic number, it is tiny!

https://en.wikipedia.org/wiki/P-adic_number

Veritasium: https://www.youtube.com/watch?v=tRaq4aYPzCc