You were given these clues to help you identify National Park Mystery Spot 31,  a building in a National Park System unit.

You can walk through this building without opening a door.
You can open a door, walk into this building, and then walk into a tree.
You can leave this building and walk into a forest.
You can walk through this forest, see trees with knees, and never touch the ground.

The answer is the Harry Hampton Visitor Center in Congaree National Park.  Renowned for its great biological diversity, giant hardwoods, and towering pines, Congaree National Park in central South Carolina preserves the largest remaining contiguous tract of old-growth bottomland hardwood forest in the United States.

Congratulations to viewmtn, who was the first to provide the correct answer. Richp39, RoadRanger, and RangerLady also got it right. Good job.

Here is how the clues lead you to the answer.

The park's main trail, the Boardwalk Trail, loops through a section of the Congaree River's floodplain forest. It consists of a Low Boardwalk that sets directly on the floodplain's surface and a High Boardwalk that is perched above it.  When you walk the Boardwalk Trail your feet never touch the ground. You see lots of bald cypress trees, which is to say that you see trees with knees.

The park's visitor center, which was opened in 2001 and named in honor of an early proponent of Congaree Swamp  preservation, is situated on the low bluff that delimits the northern edge of the floodplain. It has several distinctive features.  For one thing, this visitor center has an open-air walkway that extends completely through the building , allowing a visitor to walk from the parking lot to the boardwalk trailhead at the rear of the building. Thus, you can walk through this building without opening a door.

Since the visitor center site is heavily forested, you can leave this building and walk into a forest. If you exit at the rear, you emerge directly onto the Boardwalk Trail and walk into a floodplain forest containing a mixture of hardwoods and huge loblolly pines. If you exit at the front, you are walking in the piney upland forest that mantles the low bluff.

The Harry Hampton Visitor Center is very inviting, so you'll want to go inside.  As you enter, you see on the right an exhibit that extends the entire length of the large room and consists of a huge fallen log set against the backdrop of a magnificent forest-scene mural painted by Blue Sky.  At the far end of the lobby is an enormous (and rather real-looking) bald cypress tree whose flaring base supports a trunk that soars to the room's high ceiling. This huge tree's buttressed base is hollow, inviting visitors to go inside (where they can peer upward and see  roosting bats).  In sum: You can open a door, walk into this building, and then walk into a tree.