Caisson (lock gate)

A caisson is like a big box that can hold water out so that workers can do their jobs under the water. When people need to build something underwater, such as a bridge across a river, they use a caisson. It's like a big elevator that goes down into the water, and then the workers climb in through a little door. When they are inside, the door gets closed tightly, and water is pumped out so that they are in a dry space.

A lock gate is like a big door that opens and closes to let boats pass through a canal. It helps control the water level by keeping water levels high or low, depending on if boats are coming in or out. The lock gate has to be very strong and sturdy to hold back all the water, and it also has to move easily so that boats can go through. The caisson and lock gate works together to make sure that the workers are safe inside and the water doesn't come in while they are working, and the water level is kept to a consistent level so that boats can pass through.