So this is how it felt when the oil started flowing on the Persian Gulf. This century’s successor to the 20^th century’s great energy game is starting to unfold around us. On one side, the inland West is bursting with particularly filthy fossil fuels: Montana and Wyoming coal, North Dakota oil, the vast reserves (third-largest in the world) of Alberta’s tar sands (or, as they say there, “oilsands”). On the other side, China and Asia’s other new industrial powerhouses, thirsty for fuel and paying a hefty premium. And in between, the monkey in the middle, the rich, fragile inland seas of Washington and British Columbia, over which the stuff must travel to reach those premium markets.

Two projects have filled the headlines and drawn the protests: the campaign to run mile-long coal trains over the mountains, along Seattle’s waterfront and up the slippery shores of Puget Sound for shipment to Asia. And the Canadian firm Enbridge’s effort to build the Northern Gateway pipeline, which would pump tar-sand oil from Edmonton, across the northern-BC vastness, to Kitimat and, eventually, to fuel-thirsty Asia.

But under the cover of those two, a third transport project is taking shape, to little fanfare or alarm on this side of the border, that could affect Washington’s waters more than the others. Kinder Morgan, another pipeline giant that, unlike Calgary-based Enbridge, is based in Houston, Texas, wants to “twin” the 60-year-old Trans Mountain Pipeline that it operates between Edmonton and Burnaby, British Columbia, just across narrow Burrard Inlet from Vancouver.

The new $4-billion “twin” line will in fact be a partly new route: Trans Mountain engineering chief Jim Davies says the company will have to open up new right of way for about a tenth of the 1,150-kilometer line, where development has crowded too close to the current line to allow more construction. The new twin would also be the buster of the litter: Last year Kinder Morgan announced it would double its Trans Mountain capacity from the current 300,000 barrels a day to 600,000, then eventually settled on 750,000. It recently announced that demand was so strong it will triple capacity, to 890,000 barrels a day.

That translates to a figure of more acute concern to residents of the San Juan Islands, Olympic Peninsula, and other shores en route: Kinder Morgan projects that this will raise the number of oil tankers sailing to and from Burnaby via the Georgia, Haro and Juan de Fuca straits from five a month to 25. Other tankers carry crude and refined oil products from the refineries at Washington's Cherry Point and Anacortes and Cherry Point, which receive crude from a spur off the Trans Mountain pipeline.

Most of what they’ll carry won’t be ordinary crude. It will be a more complex, less understood concoction commonly known by what sounds like a cartoon character’s digital avatar: dilbit, short for “diluted bitumen.” Bitumen is the sticky, biologically degraded petroleum that gets steamed and pumped, at considerable expenditure of water, energy and boreal forest, out of the vast sand basins north and west of Edmonton. It’s too thick to pump whole, so it is diluted, about seven parts to three, with lighter oils. Or, more commonly now that natural gas is so cheap, with gas condensates. In a timely twist on the coals-to-Newcastle theme, Kinder Morgan will reverse the flow in a gas pipeline that now runs from Alberta to Texas to supply this diluting agent.

Just what impacts and risks this will entail is a subject of even more dispute than your usual high-stakes environmental debate. At times Trans Mountain’s industry and government promoters (including the Harper administration) and its critics (environmental and watchdog groups, First Nations and local governments in British Columbia) seem to be talking about substances as different as olive oil and Portland cement.

Pipeline opponents argue that even diluted, bitumen is too troublesome, both inside the pipes and out, to risk transporting it. Their case turns not just on dilbit’s density, but on its levels of sulfur, acidity, abrasive particles and friction-generated heat, all of which may speed corrosion in pipelines or tanker hulls.

Kinder Morgan and various authorities contend that dilbit’s acidity is no problem. Studies by Canada’s CanmetENERGY agency found that “corrosion rates were very low” in pipelines transporting dilbit, “and did not correlate with [total acid numbers]." “We’ve transported bitumen in pipe for about 30 years,” says Trans America engineer Mike Davies, “and haven’t experienced any integrity issues. It’s not corrosive and not abrasive, and the difference in acidity is not significant.”

But that difference is certainly substantial. According to one industry tabulation, dilbit is five-plus times as acidic as benchmark Mexican Maya crude. It’s no more viscous than heavy, sour Canadian conventional crudes, but it's on average about twice as acidic and a third more sulfurous. Another way of looking at it: the share of acid by weight in dilbit is a fifth to half that in commercial vinegar, a liquid customarily deemed too corrosive to store in metal.

Heat is the other big question in piping dilbit, and it’s also a controversial one. Higher heat amplifies liquids’ corrosiveness; according to an industry rule of thumb the corrosion rate doubles with each additional 10 degrees Celsius. All oil gets warmed by friction as it pushes down a pipe; the question is how much.

Tar sands opponents contend that dilbit gets much hotter than conventional crude, thanks to more abrasive grit and higher pressures needed to move it. Not so, Kinder Morgan once again insists: “Dilbit products shipped in our pipeline do not get warmer than conventional crude,” Davies declares. “We monitor temperatures within the pipeline and there is little difference between crude types. Overall temperatures do vary seasonally. In winter temperatures are typically 10 C or less, and in summer they are typically 20 C or less. There is no need to undertake any cooling measures.”

Scientific data on pipeline friction seems to be skimpy. It’s one of the questions being examined by a panel of the U.S. National Academy of Sciences deputed to determine the risks of pipeline damage from dilbit.

The pipeline industry meanwhile prides itself on not suffering any “releases of oil caused by internal corrosion from pipelines carrying dilbit” for the 10 years leading up to mid-2012. But it’s had ample mishaps of other sorts, including several on the Trans Mountain line.

In 2007 a construction backhoe broke a pipe at the Burnaby depot, sending oil pouring into Burrard Inlet and forcing the evacuation of some 50 homes. In January 2011 Trans Mountain operators ignored warning alarms for a reported three-and-a-half hours while some 25,000 gallons of oil spilled through a failed gasket at its Sumas tank farm. Canada’s National Energy Board scored Kinder Morgan for many operational failings. And last month at Vancouver (though not at Kinder Morgan’s facility) a ship crashed into Westshore Terminals, North America’s largest coal port, spilling a heap of coal and dust into the fish-rich Fraser Delta. At least it wasn’t oil, terminal officials said, trying to look on the bright side. That would be harder to clean up.