NEW YORK (CNN/Money) - If you see Spiders, Diamonds and Qubes flying around you, don't worry; you haven't been downloaded, Tron-like, into a video game.

Instead, you're probably on the American Stock Exchange, where assets with these goofy names have been trading in recent weeks like they're going out of style.

ETFs heat up Trading in Qubes, Spiders and Diamonds has been strong lately. ETF  50-day average volume  10-day average volume  Difference  QQQ  113.12 million  137.72 million  22%  SPY  49.82 million  58.25 million  17%  DIA  7.64 million  10.27 million  34%   * All data as of May 19, 2004  Source:  AMEX, CNN/Money

It's the kind of activity that often means investors feel they're on shaky ground. Could it also mean the market is about to rebound?

Exchange traded funds (ETFs) are index funds, baskets of stocks and other assets that mimic the movements of big asset groups, including the major stock indexes.

The most active ETF is the Nasdaq 100 Index Tracking Stock (QQQ: up $0.15 to $35.00, Research, Estimates), commonly called "the Qubes," which tracks the top 100 non-financial stocks in the Nasdaq. S&P Depositary Receipts (SPY: up $0.65 to $110.27, Research, Estimates), commonly called "Spiders," track stocks on the S&P 500. The DIAMONDS (DIA: up $0.24 to $100.26, Research, Estimates) track stocks in the Dow Jones industrial average.

ETFs are a relatively new asset class, but they got popular in a hurry. Individual investors like them because they let you play the broader market without having to bother with stock-picking.

Big investors like their liquidity -- unlike mutual funds, you can trade them all day, and you can sell them short, even if their prices are falling. You can't short-sell other stocks unless their prices are stable or rising.

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Activity in the two most popular ETFs, the Qubes and the Spiders, has been especially high lately. Jason Goepfert, president and CEO of Sundial Capital Research in St. Michael, Minn., has tracked the 50-day moving average in volume for both ETFs going back to 2000, and both have been exceeding their norms by a wide margin.

In fact, their excess volume has been stomping that of the underlying indices they're supposed to be mimicking. The Qubes have been especially active, with the 10-day average of their excess volume outpacing that of the Nasdaq by 27 percent at one point this week, the biggest gap in at least four years.

The last two times the gap got that big, in early October 2003 and in late March 2004, the market hit a near-term bottom and rebounded, and Goepfert thinks that could happen again soon.

"It's a sign of uncertainty," Goepfert said. "ETFs are really attractive vehicles for investors, large and small, to flee to when they're not sure of the underlying stocks or the market in general."

"Typically, uncertainty is greatest when the market is about to bottom out," he added, saying the relationship between ETF volume and market bottoms seems to have held up fairly well since 2000.

There's sure been plenty of uncertainty in the market lately, with questions about how much the Fed will raise interest rates, how things are going to turn out in Iraq, how fast inflation will return, etc., etc.

When investors aren't sure which particular stocks or sectors are going to do well or poorly in the short term, ETFs can be a safe bet, whether they want to be short or long on the market.

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"It's been harder recently to pick stocks; it's been extremely difficult on the long and short side," said Richard Suttmeier, chief market strategist at Joseph Stevens. "I've had more success in trading the Qubes."

Suttmeier also believed the approach of Friday's option expiration was causing some traders to get into the big ETFs as a way to broaden their exposure to the market while unwinding short and long positions in specific stocks.

Still, Suttmeier wasn't sure that the recent activity in ETFs was a sign that a bottom was definitely on the way, and uncertainty is not always followed by big gains -- sometimes it's followed by more uncertainty.

For example, the excess volume in the Spiders outpaced that of the S&P by a wide margin in January 2003, during the run-up to the Iraq war, and selling continued well into March.

"I think we're close to a bottom -- I think we'll have a very short-term low roughly around these levels," said Peter Green, analyst at MKM partners. "But big volume in ETFs is not a sign of a bottom. That's more of a hedging tool at this point."