Paper on AlphaZero is a large GPU cluster operated by Google, located in the United States, in the private sector. It came online around December 2017. At that moment it ranked number 2 among known clusters.
It is built on 5,000 Google TPU v1. That amounts to about 232 H100e of AI compute. The cluster draws roughly 0.88 MW. Its hardware is estimated at $51M.
Epoch AI rates the certainty of this record as likely.