They use a sub to head to the depths where they find an underground base , complete with airport runway .
With the rest of the girls recovering from the beach party incident , Aika , Eri , and Karen are left to explore the destination by themselves .
girls use a sub to head to the depths where girls find an underground base , complete with airport runway .
With the rest of the girls recovering from the beach party incident , Aika , Eri , and Karen are left to explore the destination by themselves .