What chain is Aptos built on?

Excuse me, I was wondering if you could clarify something for me. I've been hearing a lot about Aptos lately, and I'm really curious about the underlying technology behind it. Specifically, I'm wondering which blockchain is Aptos built on? Is it a fork of an existing chain, or is it an entirely new and unique platform? I'd really appreciate it if you could provide some insight into this matter. Thank you in advance for your help.