Bitcoin occasionally mines two valid blocks at the same height. The
network keeps one and discards the other. Modern monitors can log
many recent stales as they happen, but that is a recent practice,
and the active Bitcoin chain keeps no durable record of the losing
branch. Merge-mined chains preserve their own independent evidence,
reaching back to before anyone was watching. Here is how.
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1. Forks happen, stales vanish
Two miners can find a valid block at the same height at almost
the same moment. Consensus keeps just one as canonical and
drops the other. The dropped block is "stale". The active
chain keeps no durable record of that losing branch, and before
dedicated monitoring existed, most stales simply vanished.
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2. Merge mining stamps a copy
Merge-mined chains reuse Bitcoin's proof of work. Each child
block carries a
merge-mining proof
that embeds the exact Bitcoin header the miner was hashing at
that instant, even when that header is one Bitcoin will not
keep.
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3. Child chains never forget
Bitcoin drops the stale block from its active chain, but the
child chain durably records its own history. The embedded
Bitcoin header becomes independent, permanent evidence of a
block Bitcoin no longer builds on,
including stales from before anyone was monitoring.
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4. Recover and classify
This monitor scrapes the child chains, extracts every embedded
Bitcoin header, and checks each one against Bitcoin Core. A
header with valid Bitcoin proof of work that is not on the
active chain is recovered off-chain Bitcoin evidence. Known
off-chain branches are shown as stale; Core-absent candidates
become strict orphans with matching BIP34 height evidence, or
weak orphans with matching timestamp-selected nBits evidence.