Ark is a second-layer protocol called that enables users to send and receive funds without liquidity constraints. It utilizes virtual UTXOs (vTXOs) that live off the chain and have a four-week expiration. Payments are made by redeeming existing vTXOs and creating new ones. Anonymity is enhanced through obfuscation techniques. Ark Service Providers (ASPs) act as untrusted intermediaries, providing liquidity and coordinating coinjoin sessions. The protocol can interoperate with Lightning, and users can pay Lightning invoices using HTLC-nested vTXOs. Ark allows for multiple ASPs and supports multi-part payments. Transactions on Ark are credited every five seconds and settled every ten minutes, with zero-conf vTXOs enabling immediate spending.