What would happen if the Panama Canal was just a channel without locks?
In theory, a channel could have been dug down to sea level (and below, of course, so ships could use it). However, the cost of such an excavation would far outweigh the cost of putting in the locks to help raise the ships up over the height of land on the Isthmus of Panama. I don’t know the actual elevation of the height of land there, but you could probably find it by looking up the elevation of a town or city in the middle of Panama.
