why did the nyse halt trading

On July 8, the New York Stock Exchange (NYSE) shut down for nearly four hours, without warning. A "technical issue" temporarily halted trading from 11:32 a.m. to 3:10 p.m. ET.

But the unexpected closing that rattled investors was actually just one in more than 400 "special closings" in the exchange's 223-year history.

Exactly what can close the NYSE?

Here's a look at some notable special and emergency closings that put Wednesday's glitch into perspective...