Flash And Fathom. Undertow Sins, #45

She came to uncover a conspiracy. He brought her underground to use as a weapon. Tova Renn is a relentless investigative journalist chasing the story of a lifetime: an eight-year-old environmental cover-up poisoning Florida's aquifer. Her pursuit leads her to the Swallowtail Spring expedition and into the hands of Cassian Voss. Cassian is a cold, calculated cave cartographer and local dive legend.
He knows every flooded passage, lethal restriction, and lightless chamber of the subterranean labyrinth better than anyone alive. He also knows exactly who murdered his brother down there-and he has spent years meticulously planning his revenge. When a violent flash flood collapses the cave entrance, Tova and Cassian are sealed hundreds of feet underground. Trapped in the pitch-black, claustrophobic darkness with limited air and rising water, Tova's survival depends entirely on absolute trust in her guide.
But the collapse is just the beginning. As they navigate miles of submerged, deadly passages, sharing air and forced into extreme physical proximity, the line between Cassian's cold manipulation and a fierce, genuine desire begins to blur. Tova realizes her presence in the dark isn't an accident. She is the final piece of Cassian's obsessive puzzle. Now, she must decide if she can trust the hands guiding her through the abyss...
especially when she discovers the man keeping her alive might be the most dangerous thing in the cave.