The stock market fell as much as 7% on Monday morning — a decline so steep that trading was halted for 15 minutes.

Why it matters: The massive sell-off points to Wall Street's anxiety about the global economy. The steep declines come as Saudi Arabia launched an oil price war against Russia over the weekend, while the coronavirus outbreak worsened.

What's going on: Under "circuit breaker" rules, stocks resume trading 15 minutes after the halt. The halt on Monday morning was the first since the rules were implemented after the Great Recession.