However , by the late 1950s it became apparent that it would be outgrown by about 1975 .
The original plan of 1947 had been projected to be usable beyond the year 2000 .
By the late 1950s it became apparent that it would be outgrown by about 1975 .
The original plan of 1947 had been projected to be usable beyond the year 2000 .