People hold signs during a rally against hydraulic fracturing for natural gas in the Marcellus Shale region of the state at the New York Capitol in Albany on April 11, 2011. The controversial process has been a political issue in the state for many years. (Mike Groll/AP)

Fracking is based on water. A narrow shaft is dug straight down and then horizontally through a formation of shale -- flat, layered rock. Water (mixed with some chemicals) is pumped in at high pressure, and the rock fractures. (Hence "fracking," which is short for "hydraulic fracturing.") The fractures allow oil or natural gas to seep out and rise toward the surface.

There are three environmental problems with this. The problem with oil is that it releases carbon dioxide and other pollutants when burned, exacerbating climate change. Natural gas -- mostly methane -- is much, much cleaner when burned, but if it escapes from the fracking site directly into the atmosphere, it's a much more potent greenhouse gas. (And studies suggest that a lot of methane does escape from fracking sites into the atmosphere, where it traps heat.) But perhaps the biggest environmental issue with fracking is that leftover wastewater. The wastewater is usually disposed of by being injected into deep wells at high pressure, where it can possibly seep into groundwater. Those wastewater wells have also been linked to an increase in earthquakes in fracking regions.

Fracking regions are defined by the presence of the critical element: shale. So the biggest boom in production has been in western North Dakota and eastern Montana, over a shale formation called the Bakken.

The Bakken is being fracked like crazy, with North Dakota consistently seeing the biggest population growth in the country thanks to the booming industry. (It's also indirectly why gasoline prices have fallen. Middle Eastern nations have let prices fall in part in an attempt to make fracking not profitable.)

There's another big shale formation, the Marcellus, that underlies the northern part of Appalachia, extending up into Western New York. But the Marcellus is not seeing the same sort of growth, in part because it overlaps with more developed areas, and in part because of politics.

If you drive through the Finger Lakes region of Western New York, a series of a dozen long, narrow lakes carved into the ground by ancient glaciers, you'll see lawn signs for a number of political causes. This is a more conservative part of the state than New York City, so there are excoriations of legislation introducing new gun laws, for example. But there are also a number of signs for and against fracking. For, because the region needs jobs, and jobs have been the third most important byproduct of the fracking boom after oil and gas; against, because of the risks posed by fracking -- particularly from that wastewater. The purity of the Finger Lakes is one of the main industries in the region, and spoiling the lakes with leaking wastewater would be enormously harmful.

Whether or not the state should allow fracking had been a contentious issue for Gov. Andrew Cuomo for years. It wasn't until after he won reelection in 2014 that Cuomo (D) finally nixed fracking in the state, after previously suggesting that he might allow it in the economically depressed counties along the state's southern border.

With the advent of the 2016 presidential race, though, it's been reintroduced (thanks, too, to the fact that Pennsylvania, where the Marcellus Shale extends extensively, votes shortly after New York).

During an interview with MSNBC, for example, Ted Cruz, the senator from Texas running for the Republican nomination, told the network's Chuck Todd that not allowing fracking was a mistake. Responding to a question about his dismissal of "New York values," Cruz explained that he meant the bad policies of liberal New York politicians. "[F]or example, you look at the policies of Governor Cuomo where just next door in Pennsylvania they've been experiencing incredible economic boom developing the Marcellus Shale," Cruz said, "and yet knucklehead Democratic politicians here think New Yorkers don't want jobs. You've got the Marcellus Shale here. You could have thousands and thousands of high-paying jobs if the politicians would just let you develop the resources that are here, and I want to see more jobs and more economic opportunity for New Yorkers."

The reason Cuomo didn't approve fracking, though, is that a lot of Democrats live and work and vote in the state, and for a large, vocal subset of Democrats, fracking is a nonstarter because of that water pollution and because it's an expansion of the fossil fuel industry.

During the Democratic debate on Thursday night, the two Democratic contenders for the presidency had a very different debate over the process. It's worth quoting at length.

MODERATOR ERROL LOUIS: Secretary Clinton, as secretary of state, you also pioneered a program to promote fracking around the world, as you described. Fracking, of course, a way of extracting natural gas. Now as a candidate for president, you say that by the time you're done with all your rules and regulations, fracking will be restricted in many places around the country. Why have you changed your view on fracking?

HILLARY CLINTON: No, well, I don't think I've changed my view on what we need to do to go from where we are, where the world is heavily dependent on coal and oil, but principally coal, to where we need to be, which is clean renewable energy, and one of the bridge fuels is natural gas.

And so for both economic and environmental and strategic reasons, it was American policy to try to help countries get out from under the constant use of coal, building coal plants all the time, also to get out from under, especially if they were in Europe, the pressure from Russia, which has been incredibly intense. So we did say natural gas is a bridge. We want to cross that bridge as quickly as possible, because in order to deal with climate change, we have got to move as rapidly as we can.

That's why I've set big goals. I want to see us deploy a half a billion more solar panels by the end of my first term and enough clean energy to provide electricity to every home in America within 10 years.

So I have big, bold goals, but I know in order to get from where we are, where the world is still burning way too much coal, where the world is still too intimidated by countries and providers like Russia, we have got to make a very firm but decisive move in the direction of clean energy.

LOUIS: Thank you, secretary. All right, senator?

BERNIE SANDERS: All right, here is -- here is a real difference. This is a difference between understanding that we have a crisis of historical consequence here, and incrementalism and those little steps are not enough.

Not right now. Not on climate change. Now, the truth is, as secretary of state, Secretary Clinton actively supported fracking technology around the world. Second of all, right now, we have got to tell the fossil fuel industry that their short-term profits are not more important than the future of this planet.

And that means -- and I would ask you to respond. Are you in favor of a tax on carbon so that we can transit away from fossil fuel to energy efficiency and sustainable energy at the level and speed we need to do?

What's presented here is another microcosm of the differences in the approaches of the two candidates. Clinton is strategic and incremental. Sanders is dogmatic and ambitious.

Clinton's point about bridge fuels is entirely correct. At the beginning of President Obama's first term in office, the idea that transitioning from coal to natural gas as a means of fighting climate change was widely accepted. The Sierra Club was even on-board with the idea of embracing natural gas. Again, burning natural gas is much, much cleaner than burning coal.

As the New Yorker explained in a great 2011 article on the industry, this was just as drillers had figured out the key technological issue to make fracking work: the horizontal drilling. Innovations in the ability extend a well through the shale unleashed the fracking boom, which in turn led to a glut of natural gas. At the same time, fracking shifted the debate over natural gas from one about bridge fuels to one about the process of fracking itself. Apparent seepage of methane and wastewater into water supplies and water systems drew negative attention to the process, and public opinion shifted.

That's where Bernie Sanders is now. Fracking is emblematic of the problems with relying on fossil fuels generally. It is to oil and gas what Citizens United is to campaign finance -- a not-well-understood phenomenon that's become a partisan shorthand. Sanders invokes "fracking" because he knows it prompts a visceral, negative response. Clinton's description of the politics eight years ago is correct, but the politics on the subject have changed.

In New York, which votes next Tuesday, those politics sit very near the surface. It's more a complicated issue outside of New York City, where the drilling would happen and the much-needed jobs would appear. But there are an awful lot of environmentally sensitive Democrats in the city for whom the idea itself is unacceptable. Giving us three political positions: It should be allowed (Cruz), it held promise (Clinton), it can't happen (Sanders).

Fracking sits at a fascinating nexus in American society and politics, a job creator that can help or can hurt climate change and which bears other environmental risks. Which is why the debate over it takes so many forms -- and why that debate is playing out about a mile above a shale formation that's been buried under New York State for hundreds of millions of years.