BOTH it is not natural, and made by humans. - man made it is an engineering structure as well!
A structure made by a man
The Suez Canal connects those two seas.
The Suez Canal :)
The Panama Canal.
Suez Canal
Panama Canal
Suez
A rock structure is made out of any type of rock. A rock structure can be man made but is usually naturally made, caused by weather or erosion.
They are man made structures
The Saint Lawrence Seaway
Canada's tallest man-made structure is the CN Tower in Toronto, Ontario. It stands at 1,815.39 ft - the equivalent of 147 floors.