Mystery girl
One day while Chrome was out gathering minerals for the kingdom of science he was exploring a cave when he found a strange object in the cave wall not knowing what it was he went back to get Senku so that he could see what he'd found after seeing it Senku said it was a door and it was most likely for a bunker. Chrome decided he would try to open the door and when he did they found a lader leading down so they went down and found rooms full of what Senku said was non-perishable food and all sorts of equipment and in the last room was a cryocapsule being run on a water generator and in the capsule there was a girl around the same age as Senku. When Chrome asked Senku what they were going to do Senku said we'll unfreeze her