Where will we be one day, and what will we believe? How will we consider the world around us, and the people? How will we treat the known and the unknown, and possibly other intelligent lifeforms?
Whether close or distant future, something will be similar, and something… is very likely to be different.
You come upon a ruined building in the back section of a city park (or other out of the way area of the city). The ruins are fairly overgrown. All that is really standing is a doorway and its frame. If you pass through the opened door, you travel to a different world. If the door closes, there may not be a doorway back to get you back.