Across protocol is a novel bridging method that combines an optimistic oracle, bonded relayers and single-sided liquidity pools to provide decentralized instant transactions from rollup chains to Ethereum mainnet.

Across gathers liquidity pools of ERC20 tokens on Ethereum to facilitate fast withdrawals of token deposits on Layer 2 (L2) networks like Optimism and Arbitrum. The deposits on Layer 2 are one-way and are sent by the L2 network’s slow native bridge to the pool on Ethereum, reimbursing liquidity providers. This can be thought of as the liquidity providers giving a short-dated loan on Ethereum secured by Layer 2 deposits.

To bridge the tokens, relayers post a bond along with the L2 deposit details, which allow withdrawals from the liquidity pool on Ethereum after a short challenge period.