How the hell do you lose $5 billion in one week?

What were they thinking? Amaranth Advisors violated the numero uno rule of investing: never make a trade that could knock you out. Out of business, that is. The Greenwich, Conn.-based hedge fund was scrambling today to sell holdings after a wrong-way wager on natural gas cost it roughly half of its $9.5 billion portfolio.

Amaranth's brokers -- including Goldman Sachs and other big financial firms -– stepped in to help the hedge fund raise cash by liquidating some of its assets.

The trade that led to the huge loss was attributed to 32-year-old Brian Hunter, an experienced energy trader who headed Amaranth's energy desk for the past five months. His trades brought in $800 million for the firm last year, and Hunter pocketed at least $75 million in compensation, according to Trader Monthly magazine.

Hunter's downturn was as sudden as it was shocking. He was up about $2 billion as recently as the end of August, The Wall Street Journal reports. Then Hunter's trades lost $5 billion in about a week.

Hunter thrived on volatility, reaping profits on price declines and surges alike, the Journal reports. But late last week, Hunter watched with growing alarm as gas prices took a steep dive, particularly in futures contracts for delivery of gas for this coming winter.

"(Hunter) thought he knew what the market was doing," says Journal editor Phil Kuntz, who worked on the newspaper's coverage of the Amaranth debacle. "The commodities market is very, very volatile. You make a lot of money very, very quickly but you can lose a lot of money very quickly," Kuntz adds. "(Hunter) made about $1 billion in April. He lost a bunch of money in May, then he made a bunch of money back. He had a great August -– by the end of August (the Amaranth trading desk) was up upwards of 20% (year to date) but then they took a big hit in the course of a week in September."

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