The mirror is an archive of the site the way it appeared the day before Trump took office.
On Wednesday, the Environmental Protection Agency posted a mirror of its website capturing the way it looked on January 19, 2017—the day before Trump took office.
Trump has proved himself to be delete-happy when it comes to the web presence of federal agencies. He's already axed big chunks of the Department of Education and the USDA sites, and his White House website is significantly more stripped down than Obama's. The EPA's move will allow for an archive of its site to exist long after Trump eventually guts the main one.
That's not completely far-fetched—he's already nominated a climate-change denier whose heart pumps crude oil to lead the EPA, and staff members are bracing themselves for a string of executive orders that may massively reshape the agency. Trump has also, of course, said that climate change is a Chinese hoax and expressed a desire to reverse Obama's emissions restrictions, so he probably won't have much of a problem deleting the federal data the Environmental Protection Agency currently has collected on its site.
The EPA's new mirror site does have some limitations—links and some pages may break at some point, according to a warning at the top—but it will make Trump's goal to scrub the agency's information off the web a little harder, so there's that.