Add undersea cables to the growing list of flashpoints between the United States and China.

Until recently, American companies dominated this critical area of communications infrastructure — all 900,000 miles of them, snaking across ocean floors, serving as the internet’s circulatory system, transporting the world’s data, communications, financial transactions, and even military and diplomatic traffic.

China has now entered the business, with a $500 million program to link communications between Asia, the Middle East and Europe.

Nicole Starosielski, a professor of media culture and communication at New York University, and author of the book The Undersea Network, told listeners of the radio program On Point that more than 99 percent of international internet traffic is carried via fiber-optic cables running along the sea floors. “They’re integral to global communications,” she said. “We don’t have the internet as we know it today without them.”