PYRA AND THE TEKTITES #2 by Tyree Campbell & t. santitoro

After being sold as a servant, Pyra is sent to the Aquarium to tend the fish.  When artificial gravity fails, she finds herself inside the shark tank.  After a narrow escape, she and Flanagan are sent on a mission to find the Unicorn Stone and bring it back to Ichthia.  On Europa, one of Jupiter’s moons, Pyra falls into a hole and finds herself in an underground ocean that is twenty-six miles deep . . . and Flanagan can’t get to her to rescue her!