Skyline Caverns
While Skyline Caverns is not exactly in DC, it is the closest natural wonder to the Nation's Capital. When you visit the Caverns, you can take a tour of the grounds, observe the crazy tunnels, and even embark on a mini train ride! The nicest thing about making the trip out to the Caverns? The temperatures inside the caverns remain cool all summer long. Even in the middle of July, a light jacket is recommended. This may be one of the ultimate ways to escape the heat.