Wall Street's chart gurus have spotted the ominous "Hindenburg Omen" in the stock market today.

Today's sighting is the sixth one in just eight days, and that's pretty significant for folks who believe history repeats itself.

Last week, SentimenTrader's Jason Goepfert discussed the clustering of Hindenburg Omens. Here's an excerpt via UBS's Art Cashin (It's still relevant today):

With the latest market rally, the Omens are flaring up again. There have been 5 Omens triggered out of the past 8 trading sessions (your data may vary - we're using the same sources we've always used for historical data). That's actually the closest-grouped cluster since early November 2007.

It's extremely rare to see as many Omens occurring together as we've seen over the past 50 days. The last time was prior to the bear market in 2007. The time before that was prior to the bear market in 2000.

So, what exactly is the Hindenburg Omen?

As Goepfert suggests, not everyone has the same definition. But here's Wikipedia's criteria:

The daily number of NYSE new 52 week highs and the daily number of new 52 week lows are both greater than or equal to 2.8 percent (this is typically about 84 stocks) of the sum of NYSE issues that advance or decline that day (typically, around 3000). An older version of the indicator used a threshold of 2.5 percent of total issues traded (approximately 80 of 3200 in today's market). The NYSE index is greater in value than it was 50 trading days ago. Originally, this was expressed as a rising 10 week moving average , but the new rule is more relevant to the daily data used to look at new highs and lows. The McClellan Oscillator is negative on the same day. The number of New 52 week highs cannot be more than twice the number of new 52 week lows (though new 52 week lows may be more than double new highs).

Doomers will argue that this only supports their case for a major 1987-style sell-off.

But Art Cashin warns that this obviously isn't a sure thing.

"The viewers should be alert to it, but they shouldn't be panicked," said Cashin in an interview with CNBC on Tuesday. "It is a signal that says they may have a serious sell-off not they must have a serious sell-off."

The Dow is down by around 65 points in early afternoon trading.