How to make an oxygen tank in primitive times? They repurposed a car exhaust pipe, sealed and soldered it to resist 100 atmospheres, then used car parts to make a hand pump. Pumping grew extremely tough as pressure rose, and the pump broke after ten hours, yet two oxygen tanks were filled.
Soyuz and Ryusui dove and found Kaseki trapped in seabed mud. With oxygen running low, they revived strong Taiju for help. The team rescued Kaseki, but part of his stone statue was lost. Taiju retrieved the fragment underwater. Kaseki, the first petrified human, was fully revived. Senku planned to have him build a drone.