What do I deposit, and on which network?

You deposit USDC on HyperEVM — Hyperliquid's EVM chain (chain ID 999). Not Arbitrum, not Ethereum, not a balance sitting on a centralized exchange. Your wallet has to be connected to HyperEVM.

You also need a small amount of HYPE in the same wallet to pay for gas. The app prompts you to switch to HyperEVM automatically.