Cofferdam

A cofferdam is like a big wall that you build in water so you can work in a dry area. Imagine you want to build a big building on the edge of a river or a lake, but the ground is all wet and you can't work on it. You build a wall around the area where you want to build and then you pump out all the water from inside that wall.

Now the water is all outside the wall, and you have a nice, dry area to build on! You can work on the ground, pour concrete, or do whatever you need to do without getting wet. When you're done, you can take down the wall and the water will go back to where it was before.