The CBOE Volatility Index , Wall Street’s fear gauge, shot up nearly 30 percent Wednesday, its biggest daily percentage move since May, as major U.S. stock indexes turned negative for the year due to mounting concerns about Japan’s nuclear crisis.

The market fell for a third day Wednesday but investors grew overly concerned as the fear gauge rose above the psychologically significantly level of 30 for the first time since July.

“It struck me that traders were really not showing that much concern until after the first 90 minutes of trading on Wednesday,” said Chris McKhann, an analyst at stock and options website optionMonster.com in Chicago.

“Even on Tuesday, when the market sold off, most traders appeared to be selling volatility. They sold mostly put options in individual equities, indexes and exchange-traded funds on the view that the sell-off was overdone and to generate income.”

The VIX hit an intraday high at 31.28 — up 28.6 percent from Tuesday’s close. By late Wednesday afternoon, the VIX had trimmed its gain, but was still up 14.4 percent, at 27.81.

The S&P 500 wiped out gains for the year Wednesday, falling below its 2010 close of 1,257 as an escalating crisis in Japan caused market losses to accelerate.

The Nasdaq also turned negative for the year, while the Dow briefly gave up gains for the year.

By late afternoon, though, the S&P 500 had pared its loss to trade down 1.26 percent, at 1,265.72, which put it back slightly in positive territory for the year.

Earlier, the S&P 500 hit an intraday low at 1,249.05 — down 8.59 points from its 2010 close.

Option volume in the U.S.-listed iShares MSCI Japan Index exchange-traded fund rocketed higher as the underlying shares fell 3.3 percent to $9.70.

In all, about 202,000 puts and 154,000 calls traded — or 4.8 times greater than average daily volume during the first half of the session, according to Trade Alert.