Dencun — EIP-4844 'blobs' cut L2 fees ~10×

Rollups execute off-chain but must post their data to Ethereum so anyone can reconstruct and verify the chain. Before Dencun they posted it as regular calldata, competing with every other transaction for the same expensive block space. Data was ~90% of an L2's cost.

Blobs: a separate, cheaper lane

EIP-4844 (proto-danksharding) added blobs — large packets of data attached to a block, priced in their own independent fee market, and deleted after ~18 days. Rollups only need the data to be *available* long enough for anyone to check it; they don't need it stored forever.

BEFORE                      AFTER (4844)
 [ execution + L2 data ]     [ execution ]   [ blobs ]
   one crowded market          normal gas      own market,
   L2 pays premium             ...             pruned ~18d
                                               → L2 data ≈ cheap

How a blob is kept honest

A blob isn't stored in the EVM — contracts can't read its bytes. Instead the block commits to each blob with a KZG commitment, a tiny cryptographic fingerprint. Rollups prove their posted data matches that commitment, so the data is verifiably *available* for the ~18-day window without bloating Ethereum's permanent state. After the window, nodes prune it: it already did its job.

rollup ── posts blob ──→ block (KZG commitment stays on-chain)
                            │  ~18 days: data available to all
                            ↓
                        pruned (commitment kept; bytes gone)

The effect was immediate and dramatic: many L2 transactions dropped from tens of cents to fractions of a cent. It's the single upgrade that made L2s feel cheap, and it's the foundation Fusaka/PeerDAS builds on.