Caisson (water transport)

Okay kiddo, a caisson is a big box that goes underwater. It's like a giant bathtub that can hold people and things while it's being moved through water.

When people need to dig a tunnel or build a bridge in a river or a harbor, they can't just dig and build underwater. That's too hard and dangerous! So they use a caisson to make the process easier and safer.

First, they dig a big hole in the water where they want to build or tunnel. Then they put the caisson in the hole and pump the water out of it, so it becomes a big dry box. People can go inside the caisson and continue building or digging below the water level while staying dry.

When it's time to move the caisson to a new location, they pump the water back in, so it floats like a boat. Then they tow it to the new location and pump the water back out again, so they can continue working underwater.

So, a caisson makes building and digging underwater much safer and easier because people can work in a dry environment, and they can move wherever they need to go.