Wall Street watchers have been concerned for some time about the monopolizing trend among big banks. One of the most alarming developments in recent years is a buying spree in which megabanks have been gobbling up physical assets.

Matt Taibbi of Rolling Stone has delved into this story in his characteristically colorful way, shining a light on how this particular activity took off, namely through an overlooked provision in the Gramm-Leach-Bliley Act, also known as the Financial Services Modernization Act of 1999. This arcane-sounding piece of Clinton-era legislation ranks high on the list of Very Bad Ideas coming out of Washington since the 1980s. It essentially overturned Depression-era regulations that had kept the banking sector under control and opened the door for commercial banks, investment banks and insurance companies to merge their businesses.

The fine print of the bill also allowed commercial banks to dive into any activity that is "complementary to a financial activity and does not pose a substantial risk to the safety or soundness of depository institutions or the financial system generally."

So what exactly classifies as “complementary” to financial activity? When the idea came up in Congress in 1999, JPMorgan’s Michael Patterson said it was something like American Express owning a lifestyle magazine that complemented its business. No biggie.

But in reality, it has meant pretty much everything. Like, for example, oil tankers and raw materials. The result is something the public never signed off on — banks getting their mitts on entire supply chains and industrial processes. Taibbi explains how this is going down:

“Today, banks like Morgan Stanley, JPMorgan Chase and Goldman Sachs own oil tankers, run airports and control huge quantities of coal, natural gas, heating oil, electric power and precious metals. They likewise can now be found exerting direct control over the supply of a whole galaxy of raw materials crucial to world industry and to society in general, including everything from food products to metals like zinc, copper, tin, nickel and, most infamously thanks to a recent high-profile scandal, aluminum.”

Recently, something rotten occurred in Denmark, as Goldman Sachs launched its bid to buy a 19 percent stake in the national electricity provider, a deal that would give it control of key management decisions. The streets erupted in protest as Danes (some carrying images of vampire squids) raged at the idea that government ministers could have invited an American investment bank to exert so much control over the state energy grid. The deal actually set off a crisis in the Danish government.

In California, citizens found out recently that big banks have been rigging prices in the physical business interests they own. JPMorgan Chase and Barclays are accused of manipulating the delivery of electricity in California and elsewhere. Last year, JPMorgan paid $410 million to settle allegations of power market manipulation in California to the Federal Energy Regulatory Commission. Goldman Sachs, for its part, has come under scrutiny for allegedly delaying the delivery of metals from warehouses it owned so that it could manipulate prices (it has since offered to speed delivery).

Banks not only own the supply chains, they also place bets on their activity in financial markets, such as buying commodities futures. The whole thing is a recipe for corruption and conflicts of interest. Regulators, who have doing precious little about the money laundering, bribery and other scams we already know about, have been slow to address these new problems, the scope of which may be very frightening indeed.

The financial crisis taught us that big banks have turned into dangerous companies that are not only too big to fail, but, as Attorney General Eric Holder openly admitted, too big to police. Their reach continues to extend into more areas of the economy, with little public debate. President Obama has surrounded himself with economic advisors who are not inclined to rein in the banks; some of the banks even participated in the dismantling of the laws that once protected American citizens from their predation.

A lot will depend on the ability of the regulators to adapt to the changes occurring in the banking industry. Wall Street is under pressure to get rid of some commodities assets amid growing awareness of their ability to muck around with the availability of supplies and prices. The Federal Reserve is moving toward tighter restrictions on bank roles in physical commodity markets, and has issued a request for public input on the matter.

Wall Street’s push into the physical commodities markets is a brave new world of financial risk, which will be assumed, as always, by you and me. Now we can add the fear of a catastrophic pipeline explosion to the list of events that might trigger another meltdown the taxpayers will end up paying for.

Meanwhile, you can bet Wall Street is looking for the next loophole.