The SHA-1 hash
The SHA-1 hash is a broken but widely used hashing algorithm developed by the United States government in 1995. Like most hashing functions, it takes an input of arbitrary bytes and performes a number of bitwise operations on those bytes to produce a unique digest.
It is an insecure algorithm not recommended for modern use, having been superseded by the SHA-2 algorithm family.
In 2017, Google and the Centrum Wiskunde & Informatica (CWI) announced what they called SHAttered, where they generated two different PDFs with the same SHA-1 digest. Until this point, SHA-1 was already on its way out, but no proven collision had yet been generated.
According to Google, this attack took "the equivalent processing power of 6,500 years of single-CPU computations and 110 years of single-GPU computations."