Dead Reef, Blood Tide. crimson tide series, #2

The Abyss remembers. The Alpha is gone. But it isn't retreating - it's going home. After the destruction of the Indian Ocean communication array, Project Deep Watch believes humanity has bought itself time. Kate Morrison knows better. The Alpha wasn't defeated. It was summoned. And what summoned it has been waiting in the dark far longer than anyone imagined. Sonar detects impossible structures in the hadal zone - a city older than humanity, alive with dormant signals.
Including the psychic echoes of everyone the Alpha has taken. Including Sarah. Kate has already lost people she couldn't protect. She has heard their voices used against her. Now the Cassandra and the new sub Ravenna II must descend farther than any human has survived, into a mind so vast it treats the ocean floor as its nervous system. The team that goes down may not come back as themselves. If they fail, Earth will drown.
If they survive, the Abyss may never let them leave. Alpha Abyss is a pulse-pounding sci-fi thriller for readers of James Rollins, Michael Crichton, and Peter Watts - where the horror isn't what hunts you, but what you become.