Grand Central Terminal? It's a quirky spot under the vaulted ceiling right in front of the Oyster Bar & Restaurant where you can whisper into the wall and be heard by someone else in another part of the gallery.
I'd known about it forever but had never tried it and A. had never even heard of it before so during a quick walk through the station we decided to detour to the lower level and test it out. When there, two people stand on opposite sides of the tiled dome, across from each other diagonally, and just talk into the wall. I couldn't hear the whispers of others standing by the other pillars nor could I hear A. from across the busy floor, but when I stood by the pillar I could clearly hear his voice in the air around me. He was so amused!
"I wonder what the time delay is?" he asked.
So I started off counting, "1...2..."
"3!" we both yelled simultaneously.
It's immediate. Sound travels around that dome pretty fast!
Image: O is for Oyster Bar & Restaurant Whispering Gallery