Those are awesome pics of the coffer dam

Brett and coffer dam is made of interlocking panels that are hammed into the ground one sheet at a time. Once the dam is built the water can be pumped out so the building can began. Most of the time these panels are removed after the building is complete but some just can't be pulled out. I have seen times when they send a diver down to cut the panel off, this may be the reason those in the pics are still there or they were low enough after the lake filled to not be any danger and it was cheaper to just leave them in place.