A more detailed illustration of the Hagia Sophia, with a green background, showing slightly more intricate details. 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.