ExxonMobil has been running a series of ads aimed at assuring people that shale gas drilling is safe. One of those ads, a full-page spread in Monday's New York Times and Washington Post, shows what a well looks like as it extends more than a mile and a half beneath the surface. It includes a close-up of the layers of steel and cement that are supposed to protect the surrounding earth from the gas and fluids inside the well pipe.

The ad, which depicts a gas well in the Marcellus Shale, implies that these layers of protection extend all the way down the well. But in the vast majority of horizontal wells, they do not. An Exxon spokeswoman acknowledged that fact in an email.

"The ad is a graphic that compresses over a mile into 18 inches and the enlarged area depicts the casing layers protecting the fresh water aquifer," she said, adding that all of Exxon's Marcellus wells are surrounded by multiple layers of steel and cement near the surface.

The ad makes for a good moment to remind people that most states require multiple layers of casing for only a short distance underground, so they can protect shallow aquifers. After that, a well may have only one casing layer for a short way, and then no casing at all. Some wells run for thousands of feet through rock and dirt with no cement or additional steel barrier at all. Only at the very bottom are they again encased in protective cement.

Check out our gas well diagram to see which parts of a well are usually encased. The Exxon ad, while meant to be a simple summary of how a well is built, looks the same all the way down.

Even multiple layers of casing don't always protect drinking water sources. Casing and cement failures were responsible for most of the recent gas drilling accidents in the Marcellus Shale, as well as previous contaminations in Colorado and Ohio. Pennsylvania's Department of Environmental Protection recently issued its largest oil and gas fine ever to Chesapeake Energy after casing and cement failures on its wells allowed methane gas to seep into the water supply for 16 homes.

Cementing failures also contributed to the blowout of BP's Macondo well in the Gulf of Mexico last summer, the largest oil spill in U.S. history.

The Exxon ad has also drawn attention from the Natural Resources Defense Council, which campaigns for more oversight of gas drilling. The organization sent a letter to the editor to the Washington Post yesterday, disputing the ad's claim that thousands of feet of rock protect groundwater from contamination.