The area between the second and third set of locks on the Panama Canal is Gatun Lake (21 miles). It is a large artificial lake to the south of Colón, Panama.
The lake was created between 1907 and 1913 by the building of the Gatun Dam across the Chagres River. At the time it was created, Gatun Lake was the largest man-made lake in the world. Gatun Dam was also the largest of its kind.
The canal follows a clearly marked route around the lake's islands, following the deeper water south from Gatun Locks, and then east. A small "shortcut" channel, the "Banana Cut", runs between the islands, providing a slightly shorter route through the lake; this is used by canal launches and yachts to cut a little time off the crossing, and to avoid the heavy ship traffic.
The lake is also important as a reservoir of water for the operation of the canal locks. Each time a ship transits the canal 202,000 m3 (53,400,000 US gal) of water is passed from the lake into the sea; with over 14,000 vessel transits per year, this represents a very large demand for water. Since rainfall is seasonal in Panama, the lake acts as a water store, allowing the canal to continue operation through the dry season.
This picture shows cargo ships waiting their turn to go through the third and final set of locks, the Gatun Locks, before exiting into the Caribbean Sea (Atlantic).
BTW...if you go on a cruise that offers a partial transit from the east, you go through the Gatun Locks, and then turn around in Gatun Lake, before going out the way you came in.