How low do investors think oil prices could go?

Oil prices continued their steep drop on Tuesday, dipping below $48 a barrel to their lowest levels in nearly six years. But some investors are betting that crude could fall even more -- a lot more.

Traders are selling a growing number of oil futures for March delivery priced at $30 a barrel, MarketWatch reports. Those "put" options, as they are called, give the owner the right to sell an oil contract at $30. That means investors are starting to think benchmark oil prices could drop below even that level.

Oil's plunge to the prices seen this week was all but unthinkable a year ago. And for many industry watchers, the idea of oil falling below $30 a barrel is still unrealistic. Oil didn't even get that low in 2008 and 2009, when the country plunged into a deep recession and traders worried that rising unemployment and a panicked consumer would severely blunt energy demand.

This time around, the forces cutting into oil prices have everything to do with supply. The U.S. has increased oil inventories, production in Russia and Iraq is on the rise, and Saudi Arabia is refusing to cut its output for fear of conceding market share to U.S. shale-oil producers. Meanwhile, economies around the world are slowing. Japan is in a recession and Europe is on the brink, while global demand for oil is softening.

Trying to predict the point at which oil will bottom out has proven to be difficult. Oil prices are now to the point where U.S. inflation could dip into negative territory for most of this year, says Paul Dales, senior U.S. economist at Capital Economics. As energy costs drop, gas prices are expected to sink to an average of $2.25 a gallon this month.

"Unless crude oil prices rebound immediately, the drop in inflation to below zero in January is already set in stone," he wrote in a recent report.

Of course, traders betting on sub-$30 a barrel oil could make money even if it doesn't fall below that level. That's because the value of those contracts will rise as the price of oil gets closer to $30, and flipping those bets anywhere along the way could be worthwhile.