Ronin, an Ethereum sidechain developed for the hit NFT game Axie Infinity, has been targeted in a hack that saw an estimated $625 million worth of cryptocurrency drained from its bridge.

Developer Sky Mavis announced the news today, writing that the exploit took place on March 23 but only discovered earlier today. The attacker used “hacked private keys” to execute the exploit, per the team’s report, and thus was able to forge transactions to claim the funds.

All told, the attacker took 173,600 WETH or Wrapped Ethereum (nearly $597 million) and 25.5 million USDC stablecoin ($25.5 million), adding up to about $622 million worth of crypto funds as of this writing. Most of the stolen funds are still sitting in the hacker’s wallet.