3.2 One Mile Down by Jenny T Colgan
Donna and the Doctor take a holiday in the beautiful underwater city of Vallarasee.
Things have changed since the Doctor last swam through. Vallarasee is now enclosed in an airdome, with Judoon patrolling historic sites. Now, instead of tourists wearing breathing-helmets, native Fins are forced to adapt.
But leaks are trickling into the dome. The Judoon must be persuaded that disaster is imminent, or thousands will be trapped, as the waters rise...