Audit logs are supposed to be the unassailable record of what happened inside a system. But if logs can be modified after the fact, they're worthless. In this episode, we break down how startup LogChain built a tamper-evident audit log using Merkle trees. We explore the specific data structure choices—why they chose a binary Merkle tree over a hash chain, how they batch leaves to keep performance under 5 microseconds per append, and how their scheme makes it computationally infeasible to alter history without detection. Their system now powers compliance for fintech and healthcare clients, cutting audit verification time from weeks to hours. We also touch on the trade-offs: storage overhead, garbage collection of old branches, and integration with existing databases. If you're building systems that need trusted audit trails, this episode is a practical look at a deceptively simple idea with outsized impact.