Wall Street’s superstars are tumbling Monday as a competitor from China threatens to upend the artificial-intelligence frenzy that’s created a spending bonanza. The S&P 500 was down 1.7% in midday trading and heading for its worst day in more than a month.

In a matter of days, DeepSeek proved to be so popular as to climb to the top of Apple's App Store, surpassing OpenAI's ChatGPT as the most downloaded app. But for owners of Nvidia stock, and stock traders in general, DeepSeek is on track of becoming a black swan — an unexpected event with extreme consequences.

DeepSeek topped the Apple AppStore chart and sparked fears the Chinese company is quickly catching up with OpenAI's ChatGPT while costing far less.