The dry dock of Peter I.
The dry dock in Kronstadt is the first one of such a type of construction built in Russia. The works started in 1719 ( when the island Kotlin was won back during the Great Northern War) and finished only in 1752.
A dry dock is a narrow basin or vessel that can be flooded to allow a load to be floated in, then drained to allow that load to come to rest on a dry platform. Dry docks are used for the construction, maintenance, and repair of ships, boats, and other watercraft.
The Kronstadt dry dock was named after Peter I not surprisingly. Peter I designed this dock and the principle of its worked. This dry dock was the most modern one in Europein 18th-19th centuries. The draining of the dock was phenomenally fast for 18th century. After the entry of ships and the installing of them at the dock, water pumped out just for one day! It was a fantastic result for the 18th century. Analogous docks in Europe drained for several weeks. How such results were reached?
Everything great is simple. The simple principle of communicating vessels was used here. On the island Kotlin a special pool below sea level was excavated. When it was necessary to empty the dock, the water left it by gravity. Then the special pool dried up, and ships were repaired in the dry dock.
But it is not all. The dock is 2 kilometers so in the case of siege almost a whole fleet could be hidden there.
The dry dock was used until the late 20th century, but now it does not work. The dry dock of Peter I is one of the outstanding engineering monuments in Russia.