The New York Stock Exchange halted trading late Wednesday morning for reasons that remain unclear. The NYSE says the cause was an "internal technical issue."

While trading on stock markets is sometimes halted due to national emergencies  -- markets were closed for the week after the terrorist attacks of Sept. 11, 2011 -- there have been a range of reasons that exchange officials have closed markets.

It appears that the NYSE shutdown is the first time since June 1, 2005, when trading was halted at four minutes to closing "due to a systems communications problem." The market reopened the following day. A full list of NYSE closings is available on the NYSE's Web site.

In 1994, Nasdaq shut down for more than half an hour after a squirrel chewed through a power line. The outage was particularly memorable since it was the third in just a few weeks. The other problems resulted from new software and from a faulty disk drive.

It was the second time a squirrel had caused problems for the exchange. The first incident was in 1987.

Nasdaq's most recent serious outage was in 2013, when a technical failure paralyzed trading for three hours. Nasdaq later suggested that garbled data from NYSE, Nasdaq's rival, had triggered the outage.

Each of these incidents has led to controversy about whether the exchanges are up to the complicated task of guaranteeing a reliable, open market for American investors.

Yet markets in other countries have had problems as well. A software failure at the London Stock Exchange stalled trading in major companies' stocks for several hours in 2008, one of a series of interruptions. New Zealand's market, the NZX, also blamed a three-hour interruption last summer on technical problems.

Some controversial trading tactics can sometimes wildly distort prices, as happened on May 6, 2010, in the so-called "flash crash." Earlier this year, authorities arrested a trader named Navinder Singh Sarao in connection with the crash that day, although some experts say Sarao's actions couldn't completely explain the disruption.

The Securities and Exchange Commission later approved "circuit breakers" that would automatically halt trading for a quarter of an hour if prices fluctuated too rapidly. The stock market in New York has now been closed for about an hour.

You can find a history of market closings going back further in time here.