Drivechain (BIPs 300+301)
Drivechain allows Bitcoin to create, delete, send BTC to, and receive BTC from “Layer-2”s called “sidechains”. Sidechains are Altcoins that lack a native “coin” – instead, BTC must first be sent over.
Learn more about Drivechain here: http://drivechain.info
For an example sidechain implementation, see: https://github.com/drivechain-project/sidechains