Bitcoin

Submarine swap

By Paul Brock·Updated on 22-04-2026
TL;DR

A submarine swap is an atomic exchange between on-chain Bitcoin and Lightning without intermediaries, enabling direct movement from one to the other.

Submarine swaps bridge base-layer Bitcoin and Lightning without custody risk. You send on-chain BTC into an HTLC; as soon as the counterparty sends your amount over Lightning, the HTLC unlocks and the swap completes. Fails it, you reclaim your on-chain BTC after the timeout. Boltz and Lightning Loop are well-known implementations.

Example

You hold 0.1 BTC on-chain and want to top up a Lightning channel. Via Boltz you do a submarine swap: 0.1 BTC on-chain in, 0.0998 BTC to your Lightning channel out (minus service fee). Cryptographically guaranteed — either everything succeeds or nothing changes.

Frequently asked questions

What is a reverse submarine swap?

The other direction: Lightning sats → on-chain BTC. Useful to 'escape' money from full channels into cold storage. Same HTLC principle reversed.

Related terms

Lightning NetworkPayment channelCold storageSatoshi (unit)

Further reading

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