Twenty kilometres beneath Europa’s ice, a probe finds something that should not exist, and the one person who wanted this discovery most is the one who now has to protect it from her own species.

Mara has spent eighteen months alone at Calypso, a four-module habitat drilled into the ice of Jupiter’s moon, watching a probe named Orpheus creep downward at three metres a day toward an ocean no instrument has ever touched. Then, on an ordinary afternoon, the pressure readings drop to zero. Orpheus has broken through, into water that has been sealed in darkness for longer than complex life has existed on Earth.

The chemistry says the ocean is alive. Chirality, isotope ratios, metabolic signatures, four independent lines of evidence all pointing the same direction. Her mentor, the man who trained her to distrust her own hope, refuses to let her say the word until the case is airtight. Mission Control wants an announcement before it is anywhere close. And the ocean isn’t only alive, it has already been touched, because Earth’s own microbes rode down the borehole with the probe that found it.

For the first time in the history of exploration, humanity is not the one in danger. Mara has a closing window, measured in days, to decide what she is willing to lose to keep it that way.

The Warm Dark is Book Four of The Threshold Protocol, a hard science fiction series about the discoveries waiting past the edge of what we understand, and what they cost the people who find them.

Perfect for readers of intelligent, idea-driven sci-fi in the tradition of Adrian Tchaikovsky, Alastair Reynolds, and Andy Weir.