Ethereum made the transition from Proof-of work (POW) to Proof-of-Stake (POS) on September 15, 2022, at 2:45 am EST. The event was triggered by Ethereum's main net hitting "terminal total difficulty," a predetermined point at which ETH mining became effectively impossible, with the network automatically switching over to the proof-of-stake consensus mechanism.
This switch is known as the "merge.” The merge changes the way that Ethereum transactions are validated. Previously, Ethereum worked like Bitcoin: transactions were mined by a decentralized network of computers, which raced to solve mathematical puzzles and were rewarded with new coins. With the merge, Ethereum has transitioned to a consensus mechanism. The merge itself took around 12 minutes to come into effect, with the success of the event signaled by the network successfully proposing and approving new blocks of transactions under the proof-of-stake consensus mechanism. The Ethereum network missed just one block during the transition and, after 12 minutes and 48 seconds, successfully reached finality.