What Is Layer 2? Why Crypto Fees Collapsed to Cents

On 13 March 2024 Ethereum shipped an upgrade called Dencun, the base chain itself looked no different afterwards, and yet on the networks stacked on top of it transactions that had been costing a dollar or two were suddenly costing cents, some falling under a cent. It felt wrong after years of gas complaints, because nobody trusts a cheap blockchain straight away. Those stacked networks are layer 2.

This is an explainer, not financial advice, and it starts with the money because the technology makes more sense afterwards.

Blockspace is a product, and it sells out

Every blockchain sells one thing, space in the next block, and Ethereum's base layer processes on the order of 15 transactions a second whether one person wants in or half the internet does. Capacity does not stretch. So the space goes to auction, and gas fees are the clearing price: next to nothing in a quiet week, vicious once a crowd shows up.

The first famous squeeze came in December 2017, when CryptoKitties, a game about breeding cartoon cats, filled Ethereum's blocks and slowed the whole network down for everyone else. The NFT mints of 2021 ran the same squeeze at far bigger scale, and people were sometimes paying more in gas than the item they were minting was worth, because the auction was doing exactly what it was built to do and the crowd kept bidding anyway.

What a layer 2 actually does

A layer 2 runs your transaction on its own machines, off to the side, and every so often it compresses everything it has processed into one record and posts that record back to Ethereum, so thousands of transactions land on the base chain as a single receipt and everyone in the bundle splits one bill for one slot of blockspace instead of outbidding the crowd for a slot each. The saving is not clever, it is just bulk.

Dencun pushed the price down again by adding blob data (EIP-4844), a cheaper class of storage built for exactly this compressed cargo. That is the whole reason fees fell to cents in March 2024.

Security is the finer print. Because the results land on Ethereum, a layer 2 inherits most of the base chain's security, though not all of it, and the two places the inheritance runs out are the proof system and the bridge.

Rollups, and the two kinds of proof

Rollups are the main family: bundle thousands of transactions, post the batch to Ethereum, stand behind the batch with a proof, and the proof comes in two designs. Optimistic rollups, the design behind Arbitrum and Optimism, both open to the public since 2021, post first and leave a challenge window in which anyone can submit evidence of fraud, and that window is the whole reason withdrawing back to Ethereum by the official route can take days. Validity rollups, the ZK family, go the other way round: a cryptographic proof up front, nothing left to challenge.

Third-party bridges will sell you a shortcut past the optimistic wait, for a fee and a risk of their own.

Lightning, briefly

Bitcoin runs the same play for payments: the Lightning Network, usable since 2018. Open a channel, transact off-chain as often as you like, settle the net result back to the base chain later, because base-chain blockspace is too dear for coffee money.

The Polygon asterisk

Polygon started in 2017 under the name Matic, and the busy PoS chain that millions of people mean when they say Polygon is a sidechain, which runs its own validator set and looks after its own security rather than inheriting Ethereum's, so if those validators ever failed the base chain could not step in to help.

The wider Polygon operation also builds zkEVM rollup technology, which does post proofs back to Ethereum, so whether Polygon counts as a layer 2 depends on which Polygon product you mean, and plenty of people moving small everyday amounts have clearly decided they can live with the difference.

Bridges are where the money died

In February 2022 attackers took about $320 million out of Wormhole, and a month later the Ronin bridge behind the Axie Infinity game lost more than $600 million, still one of the biggest thefts crypto has produced. Moving coins between layers takes a bridge, a contract that locks funds on one side and issues a matching version on the other, so every big bridge ends up sitting on a pot of locked coins with nothing but software guarding it, and pots that size get noticed.

None of this is bad luck, because a layer 2 keeps fees down by doing its business one step away from the base chain, and coins have to cross that step somewhere, so the locked money piles up right at the crossing, and a contract holding nine figures is a target whether its authors think so or not.

Using one without getting caught out

Three things bite newcomers, and none of them is the clever cryptography. The big one: the same token can exist on several layers at once, so wallets and platforms ask you to pick a network from a menu, and coins delivered to a network your wallet is not showing look exactly like money gone. Sometimes they are. Gas is the next surprise, because each layer charges fees in its own coin, ETH on Arbitrum and Optimism, POL, the coin formerly called MATIC, on Polygon's PoS chain. Then the waiting: moving funds from an optimistic rollup back to Ethereum takes days by the official route, or a bridge fee by the fast one.

Buying through an on-ramp puts the same network menu in front of you at delivery. Banxa has run fiat-to-crypto plumbing since 2014, with more than 100 payment methods across 100-plus countries in the markets it serves, and card orders typically complete within about 10 minutes of issuer approval. The network check before you pay takes ten seconds, so do it in that order.

The economics have held up since Dencun and the fees really are pocket change now, which leaves the network menu as the main place people still lose real money, so read it twice before you send anything, however small the transaction has become.