It's block 756951: https://mempool.space/block/0000000000000000000000005d6f06154c8685146aa7bc3dc9843876c9cefd0f. It has a difficulty of more than 50 quintillion (50 million trillion). For comparison, right now difficulty is 127 trillion. That means it's around 393,000x more difficult to solve.

Imagine that with 900 EH/s, the network has a little over 2% chance of mining a block at that difficulty in one year. With current hashrate, it's expected to find such difficulty only once per 50 years.