Across is an optimistic bridge that combines a novel oracle system, bonded relayers, and single-sided liquidity pools to provide near-instant transfers across chains.
Across is a bridging solution engineered and powered by the UMA protocol's optimistic oracles, which allow smart contracts to use off-chain data in an economically secure manner. Across facilitates instantaneous transfers between Layer-2 rollups and the Ethereum mainnet. It was originally designed to speed up the transfer of assets from L2s back to L1 as native Optimistic bridges like Optimism and Arbitrum need to wait a 7-day dispute period before finalizing transactions. Across incorporates several novel designs including single-sided liquidity pools, bonded relayers, and the use of optimistic oracles. Liquidity providers on Across allow near-instant transfers from L2 to L1 and are paid a fee for each transfer. Relayers are the entities that report information about transactions between chains. Their main job is to report transaction details to UMA's optimistic oracles. Relayers must post a bond to use the oracles as a safety measure against malicious actions. After a relayer attests to a transaction and there are no disputes, withdrawals from the liquidity pool are unlocked. There are two types of bridging actions: fast and slow relays. Fast relays occur when a relayer provides funds to a bridge user and gets reimbursed later from the liquidity pool after the dispute period. Fast relays are near instantaneous. Slow relays occur when a relayer claims that a particular deposit happened on L2, but it did not have the necessary funds at the time to front the transaction. In this case, the slow relay can take up to two hours.
Across charges two types of bridge fees: the liquidity provider (LP) fee and the relayer fee. The LP fee is paid to users who provide passive liquidity and is utilized by the system to refund relayers. The relayer fee is paid to incentivize relayers to relay user cross-chain transfers quickly. The LP fee depends on the transaction size and utilization rates and typically ranges between 0.06% to 0.12% of the transaction. The relayer fee is chosen by the user to incentivize relayers to relay their transactions. Slow relay fees are optional fees that a user pays for a relayer to acknowledge their transaction. Instant relay fees incentivize relayers to instantly accept and front the transaction.
You can make money by depositing liquidity into single liquidity pools for native assets to earn LP fees. More advanced users can also run their own relayer node to earn relayer fees.