Caisson (lock gate)
![Ship caisson at Droogdok Jan Blanken at Hellevoetsluis](/Images/godic/202502/07/Hellevoetsluis_-_Droogdok_Jan_Blanken_-_schipdeur_dok_droog5411.jpg")
![Graving docks at Birkenhead, closed by a variety of ship caissons and floating (sliding) caissons](/Images/godic/202502/07/Dkbkpl325411.jpg")
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A caisson is a form of lock gate. It consists of a large floating iron or steel box. This can be flooded to seat the caisson in the opening of the dock to close it, or pumped dry to float it and allow it to be towed clear of the dock.