The underground tunnel between the East and West buildings of the
National Gallery of Art.  This is the part of the National Gallery that
I remember the most from when I was a kid.  I remember sitting at
the cafe; I remember the moving sidewalk; most of all, I remember the
angled waterfall and the vertically flowing air right in front of it. 
It's odd what sticks in one's mind.
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Incidentally, the moving sidewalk bounces a lot when one walks on it.  
It's springy. (I didn't remember this fact.)