Immutable is a [layer-two](https://coinmarketcap.com/alexandria/glossary/layer-2) blockchain with zero [gas](https://coinmarketcap.com/alexandria/glossary/gas) fees, where operators can set their own trading fees. In contrast to other scaling solutions to Ethereum, a 51% attack on Immutable is unfeasible, as it is not a centralized side chain but benefits from inheriting the native security of the Ethereum blockchain. Immutable uses zk-rollups, meaning assets are traded on the second-layer blockchain, but the validity proof of a transaction is stored on the layer-one blockchain, in this case Ethereum.
Immutable chose to stay on Ethereum as a first-layer solution because the team considers Ethereum best to represent the philosophy of trustless, decentralized asset ownership. In the team’s words, Ethereum has always prioritized centralization over short-term scalability upgrades, even though it has suffered from problematic gas fee spikes and network congestion, thus still remaining the main blockchain for NFT projects to build on.