PYRA AND THE TEKTITES 6: DANGER UNDER EUROPA by Tyree Campbell

Pyra has paid off her debt to Mark the Shark, who now wants her to work for him. She likes her independence, and returns to Europa with Kathi, whose parents have allowed her to travel in space for a while. Also with her is Jahn Gassen, who delivered goods to Pyra’s ship and was accidentally taken into space.

Pyra returns to Europa to study under Dr. Mairead O’Rourke. She receives a telepathic message from the morrikaru, the blind seal-like creatures who live in the ocean under the crust of Europa. The morrikaru report strange sounds coming from the walls around the ocean. Pyra and Kathi go to investigate, and find a crack in the walls.

What if the surface of Europa should collapse? Uh-oh.
 
IN THE PYRA AND THE TEKTITES SERIES
Pyra and the Tektites
Aquarium in Space
Smugglers!
Space Girl
Lost on Earth