A rendering of Trinity River Park in Dallas, Texas. Michael Van Valkenburgh Associates

Dallas, Texas, is getting a lot greener.

The city is building a 10,000-acre nature district - nearly 12 times as large as Manhattan's Central Park - along the Trinity River. Featuring plenty of walkways, sports fields, trees, and other flora, the site will become one of the largest urban parks in America.

In late October, Annette Simmons, the widow of the billionaire Harold Simmons, donated $50 million toward 285 acres of the ongoing project. Set to be complete by 2021 and costing $250 million, this portion will be called the Harold Simmons Park. Dallas Mayor Mike Rawlings said he expects other private donations to fund the rest of the Harold Simmons Park, according to Dallas News.

It will become part of the larger nature district, called the Trinity River Project, that began in the early 2000s. As of June 2015, the city had spent over $609 million to build trails, a bridge, a horse park, a golf course, and a community center in a 6,200-acre forested area. In the future, there are also plans to build shops, restaurants, housing, and offices near the river.

Check out what the Harold Simmons Park portion, designed by Michael Van Valkenburgh Associates, will look like: