A ZK rollup (zero-knowledge rollup) generates a cryptographic proof showing that every transaction in a batch was executed correctly, and posts the proof along with the new state root to Layer 1. The L1 verifies the proof (cheap on-chain) and accepts the state immediately. Unlike optimistic rollups, there is no challenge window: state is final as soon as the proof is verified.
Generating a ZK proof of an EVM execution is expensive (minutes to hours of compute) but verifying it on L1 is cheap (a few hundred thousand gas). The asymmetry is the whole game: prove off-chain, verify on-chain. Modern ZK rollups use proof systems like Plonky2, Halo2, and STARKs to make proving fast enough for production.
EVM-equivalence (running unmodified Ethereum bytecode) is hard for ZK rollups; older designs used custom VMs. Newer "type 1" and "type 2" ZK rollups (Linea, Scroll, Polygon zkEVM) approach full EVM compatibility, making them drop-in replacements for L1 from a developer perspective.
ZK rollups are the long-term endgame for Ethereum scaling: instant finality, no withdrawal delays, mathematically guaranteed correctness. The main historical blockers (proving cost, EVM compatibility) are falling fast.
The DeFi TVL endpoint tracks total value locked across all major L2s, including the largest ZK rollups (zkSync, Linea, Starknet, Scroll).