Okay, seriously, I know I’m supposed to be smarter than this, but I don’t get it. So one side is higher than the other, which is why you need all those locks to take a ship from one end of the canal to the other. Why, once they built the canal, didn’t the taller ocean equalize with the shorter one? And why did it never happen in the past? Frankly, how can one ocean be higher than another considering that they all communicate one way or another?
Actually, the locks are there because the middle of the canal is higher than the ends. Gatun Lake is 85 ft. above sea level.
You need the locks, not because of any difference in level between the oceans, but because the middle part of the canal is made up of an artificial lake, Lake Gatun, which is 85 feet or so above sea levels. Ships enter one end of the canal; are lifted to lake level via locks; pass through Lake Gatun; and then are lowered back down to sea level with the other set of locks.
A Cecil column on the underlying misconception in this question.
At Panama, the Pacific is slightly higher than the Caribbean.
The Pacific is less salty (and hence less dense) than the Atlantic because water evaporates in the tropical Atlantic, leaving the salt, and then the water-vapor-laden air passes over the isthmus driven by the trade winds. The vapor condenses over the Pacific and falls as rain, freshening the water.
As the link indicates, because of this even if a sea-level canal were built locks would still be necessary. Also, Pacific tides are much higher and out of sync with those on the Atlantic side.
The question works for many, if not most canal systems, like the Erie Canal, which is basically downhill all the way from Buffalo to Albany.
The simplest answer is that the water is flat up until the lock, where it hits a wall. When the lock opens, the water spills out, but only when the lock is open. The canal is an artificial basin along each step that is built to retain a given height of water.
Here’s a nice profile picture for the OP. The vertical scale is exaggerated but one gets the idea.
Here is a cool timelapse of a ship traversing the canal. You can see how the locks operate.
Well, not really.
A lock has two gates arranged so that the water level difference tends to close them, preventing - or at least greatly restricting - water flow. The area between the gates can be filled from the higher water and drained to the lower water, but this is done with pipes and valves, not by opening the gates. One of the gates is opened when the water level between them equalizes to one of the two levels.
Unrelated to this question, but my favorite fact about the canal is that the Atlantic end is West of the Pacific end.
Another good one is the fact that the canal is actually east of Miami FL.
Of course, it’s pretty much due south - but in terms of longitude, about 15 miles east.
My favorite fact* is that, before they controlled them, mosquitoes were thick enough to, at times, put out a candle. :eek:
*according to David McCullough
Ahhh, that makes sense now. I too always wondered about this. So it’s not that the locks are there because the two oceans are at different heights, it’s there so the canal didn’t have to be dug nearly as deep.
Note that the Suez Canal has no locks and water does flow freely from the Med to the Indian Ocean.
Assuming for a moment that they had built it to be level, there would probably be a tidal current from one end to another, which would change with the tides, and you might get a buildup of silt near the entrances
And if all the locks were all just suddenly removed somehow, the lake would drain, and wouldn’t be navigable, correct?
Correct - the bottom of Lake Gatun is definitely not below sea level.
That is VERY cool - thanks!
You would have to use prairie schooners 
Or call in the Aussies.