PLATO was developed at the University of Illinois Urbana-Champaign. wikipedia
I can remember learning of their ability to support thousands of terminals on the same CDC 6500 mainframe for which we could only support hundreds.
I learned from them the computational power of memory, especially small blocks of memory allocated to each terminal, operating as a cache for high-frequency operations.
I also first saw a graphics terminal deployed more than one to a computer. We were still using model 33 teletypes sending text-only back and forth.
Mike Caulfield's colleagues recall Personal Memories of PLATO in his Hidden History of Online Learning project.