The ‘Long Wall’, also known as the ‘Great Walls’, is a set of military fortifications built, destroyed and rebuilt several times and in several places between the 3rd century BC and the 17th century to mark and defend China's northern border. It is approximately 6,260 km long!
Here's one end that flows into the sea.

https://www.klm.fr/travel-guide/inspiration/the-world-s-longest-wall
Difficulty ***