The Berlin Wall
The Pyramids of Giza
Slavery
The identity of Jack the Ripper
The atomic bombs
The cause of the Mary Celeste's mysterious desertion
Apartheid
should not have been allowed to exist in the United States
should not have been dropped on Hiroshima and Nagasaki
might have been built by aliens
should not have been constructed during the Cold War
might never have been discovered
could have been a giant squid.
should not have been practiced in South Africa
The disappearance of Amelia Earhart may have been caused by engine failure.