Alaska already has one famous 800-mile pipeline, which brings oil from the Arctic to tankers waiting in the south. Now the state is vigorously pursuing a plan to build a second one to carry the far north’s vast, untapped reserves of natural gas to a growing China.

The pipeline proposal is as ambitious as its predecessor, the Trans-Alaska Pipeline System. Starting at Prudhoe Bay, on the Beaufort Sea, the proposed 42-inch-diameter pipeline would extend south for 825 miles to Nikiski, on the Kenai Peninsula, southeast of Anchorage. The price tag for such a project is bogglingly high—an estimated $43.4 billion, including a $9.3 billion contingency fund. It’s unclear whether the project will find financial backers to help foot the bill, though.

The idea is controversial for other reasons, too: building a natural gas pipeline would extend the world’s reliance on fossil fuels and lead to more climate change-causing gasses, critics point out. It also could spur future energy development in the Arctic.

What’s not in dispute is that an enormous amount of natural gas lies beneath the nation’s Arctic—roughly 200 trillion cubic feet of so-called conventional natural gas, counting the gas beneath the Beaufort and Chukchi seas, says David Houseknecht, senior research geologist for the U.S. Geological Survey. (For comparison, total U.S. consumption was 27 trillion cubic feet last year, according to the U.S. Energy Information Agency.)

With no pipeline to bring gas to market, the gas has been “stranded,” says a spokesman for the gas line effort. A pipeline would change that, says USGS’ Houseknecht, “There’s no doubt that a pipeline would make the gas resources economically viable to develop, because right now the gas’ worth is economically zero, or nearly zero,” he says. What’s more, once a main gas pipeline is in place, more new gas exploration, and pipelines connecting the new wells the main pipeline, would appear over time, he predicts—rather like branches on a Christmas tree

That worries many environmentalists. “This is a horrible idea,” Nathan Matthews, a staff attorney for the Sierra Club, told Outside. In a filing with FERC, Matthews wrote that “Sierra Club members everywhere will be affected by the greenhouse gases emitted by the increased natural gas production induced by the project.” The Center for Biological Diversity wrote in a similar filing that “the Project will result in increased natural gas drilling in the Arctic….increasing air pollution, diminishing wildlife habitat, and exacerbating climate change.” A completed pipeline would make several river crossings, and would pass for 28 miles beneath Cook Inlet, which is home to endangered beluga whales.

The price tag for such a project is bogglingly high—an estimated $43.4 billion, including a $9.3 billion contingency fund.

Much of the natural gas lies in areas where there already are existing wells, says Houseknecht, pointing to Prudhoe Bay and Point Thomson, the latter about 60 miles to the east near the Arctic National Wildlife Refuge. Tapping that gas wouldn’t require developing a lot of new ground. Other areas where large amounts of natural gas have been found, however, don’t have any significant development. For instance, exploratory wells over the years in the foothills of the Brooks Range in the National Petroleum Reserve-Alaska, or NPR-A, have found significant gas there, he says.

Not all environmentalists see the natural gas pipeline in a negative light. “I don’t think the environmental ramifications of the gas line are as great as others seem to think if it is built with minimal leakage and follows existing rights of way,” says Lois Epstein, Arctic program director for the Wilderness Society and an engineer who has served on the federal government’s oil pipeline federal advisory committee for a dozen years. “I don’t see NPR-A development happening for gas for decades, if ever.” Epstein added that the pipeline would be able to bring cleaner natural gas to communities such as Fairbanks, which now have horrific air quality in winter due to wood-burning.

Ultimately, the outcome will come down to money. The new pipeline would provide gas to China, whose demand for natural gas is expected to surge in the coming decades, according to the Energy Information Agency. Alaska hopes to attract Chinese investors to the project. Last November, Alaska Governor Bill Walker went to Beijing and signed a deal between his state and three Chinese entities, including a major state-owned petroleum company and the Bank of China, to advance the pipeline. The deal was non-binding, but the groups hope to have a formal agreement on financing and investment by year’s end, according to a spokesman for the Alaska Gasline Development Corporation, which was formed by the state of Alaska to pursue the project.

Even with Chinese backing, the pipeline might not make sense financially. An August 2016 report by Wood Mackenzie, an energy consulting firm, found that Alaska’s LNG project “one of the least competitive” of several projects that could serve northern Asia with natural gas. A big reason was its huge construction costs. (The report also suggested ways the project could boost its competitiveness.)

Meanwhile, the cost of renewable energy—wind, solar—continues to fall. Natural gas is often touted as a “bridge fuel” to a cleaner future. It’s unclear, however, how long that bridge is.