Etna is a borough in Allegheny County in the U.S. state of Pennsylvania, along the Allegheny River, opposite Pittsburgh. It flourished in the past when it was the site of rolling mills, furnaces, steel mills, galvanized-pipe works, and other manufacturers. In 1900, 5,384 people lived in Etna. In 1910, 5,830 lived there, and in 1940, 7,223 lived there.