Department stores across the country account for more than 350 million square feet of mall space. And that number is really scary to American mall owners.

Department stores are under attack as consumers instead opt to increasingly shop online or head to discounters and off-price retailers like TJX's TJ Maxx. Brands like Sears and Bon-Ton have filed for bankruptcy this year, closing hundreds of locations. But even those still surviving like Macy's or J.C. Penney are evaluating their massive real estate portfolios, which analysts expect will lead to more store closures.

For the malls and their owners, the fear is more and more of these massive structures will have cornerstone department stores go dark — which inevitably reduces foot traffic in the malls. There are younger digital brands like Casper and Untuckit looking to grow, but not with the hundreds of stores of more than 50,000 square feet in size.

For mall owners, this report paints a dire picture. Commercial real estate advisory firm Green Street Advisors determined that all the department stores still open in the U.S. could fill 350 average-size malls — by themselves. That's just how many of them are left, and how big they are. Macy's has the largest share of square footage in malls, followed by Penney, Sears, Dillard's and Belk, based on Green Street's analysis.