MASSENA, N.Y. — The hulking aluminum plant in this northern border town is starting to spew heat and noise again, four years after Alcoa shut it down. But now the hot hum comes from thousands of Chinese computer servers whirring away 24 hours a day for a very modern purpose: producing Bitcoins and other digital currencies.

The crackerbox-size machines stacked inside rusty cargo containers are powered by the same cheap source of electricity once used to extract aluminum from ore. They represent the first stage of an obscure company’s plan to convert the 60-year-old smelting works into the world’s biggest cryptocurrency mine.

“The size is overwhelming,” said Prieur Leary, as he led a tour of the 1,300-acre site formerly known as Alcoa East. “Maybe we’re a little bit crazy.”