ABOARD THE CELEBRITY MILLENNIUM — Have you any idea what it feels like to be four stories up at ground level? Right there on the other side of the railing is grass. Also railway tracks. And trains. And the bottom of the hull of a huge ship, just 50 yards away. And workers, at eye level.
This is life in the locks. Specifically, the Miraflores Locks of the Panama Canal, the final barrier to the Pacific Ocean. You look down through the foot or so of space separating your cruise ship from the land, and there’s a narrow four-story drop to the water.
That’s sea level.
The ship, in this case the Millennium, slips out of the lock seamlessly as the “ground” drops away and it enters the last stage of crossing the Isthmus of Panama. It enters the waters of the Pacific Ocean, which means it’s salty…and therefore thicker. 
For ships going the other way, this can be a problem. In salt water ships are more buoyant than in the fresh water of the canal. So if their load is too heavy, the hulls go too deep in the canal and they have to stop and unload part of the cargo, to be shipped by rail and re-loaded on the other side.
Just one of the quirks of the Panama Canal, where ground level is not always what it seems. See for yourself, any time, from the Miraflores Locks web cam.
That’s it…we’re done.