The saemangeum seawall is a really, really big wall that was built to keep the ocean water from flooding an area in South Korea. Imagine you are at the beach and there's a big wave coming towards you. The saemangeum seawall is like a super big sandcastle wall that's meant to keep the wave from getting past it and flooding an entire area.
This seawall is not just any wall -- it stretches really far and is actually the longest seawall in the world! It's almost as long as the distance from the Statue of Liberty in New York to the Eiffel Tower in Paris. That's really, really long!
Lots of people worked really hard to build this wall because they wanted to create more land area for farming and other activities. Imagine if you had a really big room, but you wanted to make it even bigger so you could put in more toys, pillows, and clothes. That's what they did with the land area. They wanted to make it big enough to grow more crops and raise more animals for everyone to eat.
Now, the saemangeum seawall is kind of like a big protector for the land behind it. It makes sure the water and the land stay where they're supposed to be. Isn't that really cool?