Definition
A submarine swap is a trustless exchange between on-chain Bitcoin and off-chain Lightning balance. It is a specific kind of atomic swap that bridges the two layers without custody or counterparty risk, making two transactions conditional on each other so that they either both execute or neither does.
How it works
The mechanism reuses the same hashlock that secures ordinary Lightning payments. A user and a swap provider agree on a payment hash; the on-chain leg is locked in an HTLC and the off-chain Lightning leg is locked to the identical hash. When the preimage is revealed to claim one leg, it automatically unlocks the other. Neither side can cheat: the timelock refunds the payer if the swap stalls.
Why operators use them
Submarine swaps solve real liquidity problems. A "loop out" sends Lightning balance and receives on-chain Bitcoin, draining local balance to create receive-side room. A "loop in" does the reverse, refilling sending capacity from an on-chain UTXO. They also let users top up a Lightning wallet from on-chain funds, or cash out to chain, without manually opening or closing channels.
Submarine swaps are a primary tool for acquiring inbound liquidity and rebalancing the channel capacity of a busy node.
In Simple Terms
A submarine swap is a trustless exchange between on-chain Bitcoin and off-chain Lightning balance. It is a specific kind of atomic swap that bridges the…