What you learned in school
Geography and oceanography textbooks described ocean floors as vast, flat plains covered in mud from continental erosion. Students learned that the deep ocean was essentially uniform and boring compared to land geology. Before sonar mapping, it was presumed that except for occasional hills, the seafloor was generally featureless and monotonous - a muddy desert stretching endlessly across ocean basins.
What we know now
Geography classes taught that ocean floors were essentially flat, muddy plains with little topographic variety. Students learned that all the interesting geology was on land. The deep ocean was considered a vast, uniform environment. Sonar mapping during and after World War II revealed dramatic underwater landscapes: mid-ocean ridges, deep trenches, underwater mountains, and complex geological features. The ocean floor is actually more geologically active and varied than most land surfaces, with new crust being created at ridges and destroyed at trenches.