A basic illustration of the Hagia Sophia, set against a simple blue background. On Dec 8, 2019, the Istanbul hard fork optimized the gas costs of certain actions in the Ethereum Virtual Machine (EVM) and improved resilience against denial-of-service attacks. It made Layer 2 scaling solutions based on SNARKs and STARKs more performant. The fork enabled interoperability between Ethereum and Zcash. Additionally, it allowed smart contracts to introduce more creative functions.