Synthetix has turned on multicollateral margin on Ethereum Mainnet, with ETH now the first non-USDT asset traders can post against USDT-settled perp markets. The setup lets users keep spot ETH exposure while using it as account collateral, though ETH is marked at live index price with a haircut, USDT balances can go negative, and the protocol can auto-convert ETH if debt breaches limits. Synthetix is pitching this as a way to tap more than $100B in idle ETH and says capturing 10% of average monthly derivatives volume would mean $300B-$500B of monthly L1 perps flow.

TLDR by @Benthic