Abstract: A mobile submersible caisson is employed to drill for and recover petroleum products from subsea wells and to deliver the products to shore or to vessels at the well site. The well drilling technique applies to water depths beyond 300 ft. and in areas where severe winds, waves, spray and frigid atmosphere can cause long downtime or make it impossible to maintain the positions of surface platforms for extended periods of time. Caissons containing drilling, completion, workever, separation, pumping and compressing equipment are towed to sea in the caisson in the horizontal position. At the work site the caisson is erected, connects an attached wire cable to a platform placed on the sea bottom, and is submerged and winched down. The caisson mates and becomes sealed with the platform, providing a one atmosphere working space for the drilling crew. Water between the caisson and the platform is pumped out so this area can also be used as a work area.