WHEN THEY REACHED the ledge, Mia was sure she'd signed their death warrants.

The cliff dropped more than eighty feet. At the bottom stretched a nightmarish version of the Grand Canyon: a river of fire cutting a path through a jagged obsidian crevasse, the glowing red current casting horrible shadows across the cliff faces.

Even from the top of the canyon, the heat was intense. The chill of the River Cocytus hadn't left Mia's bones, but now her face felt raw and sunburned. Every breath took more effort, as if her chest was filled with Styrofoam peanuts. The cuts on her hands bled more rather than less.

Assuming they could make it down to the fiery river, which she doubted, her plan seemed certifiably insane.

"Uh . . ." Percy examined the cliff. He pointed to a tiny fissure running diagonally from the edge to the bottom. "We can try that ledge there. Might be able to climb down."

He didn't say they'd be crazy to try. He managed to sound hopeful. Mia was grateful for that, but she also worried that she was leading him and Annabeth to their dooms.

Wouldn't be the first time, a voice in her head said. Shut up, she told herself.

Of course if they stayed here, they would die anyway. Blisters had started to form on their arms from exposure to the Tartarus air. The whole environment was about as healthy as a nuclear blast zone.

Percy went first, then Mia then Annabeth. The ledge was barely wide enough to allow a toehold. Their hands clawed for any crack in the glassy rock. Mia had ripped off part of the sleeves of her hoodie ( man, she really should've put that on earlier, but it was sadly wet from the Cocytus ) and used the cloth to wrap her bloody palms, but her fingers were still slippery and weak.

A few steps below her, Percy grunted as he reached for another handhold. "So . . . what is this fire river called?"

"The Phlegethon," she said. "You should concentrate on going down."

"The Phlegethon?" He shinnied along the ledge. They'd made it roughly a third of the way down the cliff — still high enough up to die if they fell. "Sounds like a marathon for hawking spitballs."

"Please don't make us laugh," Annabeth said.

"Just trying to keep things light."

"Thanks," she grunted, nearly missing the ledge with her bad foot. "I'll have a smile on my face as I plummet to my death."

Mia nearly snorted. She loved them.

They kept going, one step at a time. Mia's eyes stung with tears from the heat. Her arms trembled. But to her amazement, they finally made it to the bottom of the cliff.

Annabeth had stumbled when they'd gotten to the ground, and Percy caught her. Red boils had erupted on their faces, so they looked like smallpox victims.

Mia's own vision was blurry. Her throat felt blistered, and her stomach was clenched tighter than a fist.

We have to hurry, she thought.

"Just to the river," she told them, trying to keep the panic out of her voice. "We can do this."

They staggered over slick glass ledges, around massive boulders, avoiding stalagmites that would've impaled them with any slip of the foot. Their tattered clothes steamed from the heat of the river, but they kept going until they crumpled to their knees at the banks of the Phlegethon.

"We have to drink," Mia said.

Percy swayed, his eyes half-closed. It took him a three-count to respond. "Uh . . . drink fire?"

"The Phlegethon flows from Dad's realm down into Tartarus." Mia could barely talk. Her throat was closing up from the heat and the acidic air. "The river is used to punish the wicked. But also . . . Thany called it the River of Healing."

"Thany?" Annabeth repeated.

"Thanatos, when I asked him and Dad about it." Mia swallowed, trying to stay conscious. "The Phlegethon keeps the wicked in one piece so that they can endure the torments of the Fields of Punishment. I think . . . it might be the Underworld equivalent of ambrosia and nectar."

Percy winced as cinders sprayed from the river, curling around his face. "But it's fire. How can we—"

"Like this." Annabeth thrust her hands into the river. She cupped the fiery liquid in her palms and raised it to her mouth.

Her face scrunched up. Then, suddenly, she collapsed, gagging and retching, her whole body shaking violently.

"Annabeth!" Percy grabbed her arms and just managed to stop her from rolling into the river.

"Holy shit," Mia breathed out. "You actually did that."

The convulsions passed. Annabeth managed to sit up. The blisters on her arms were starting to fade.

"It worked," she croaked. "Mia, you're a genius. You guys have to drink."

"I . . ." Percy's eyes rolled up in his head, and he slumped against her.

Mia let Annabeth deal with him as she plunged her hands into the river. On first contact, the fire wasn't painful. It felt cold, which probably meant it was so hot it was overloading her nerves. She drank the fire liquid and immediately regretted it.

She expected a taste like gasoline. It was so much worse. Once, she'd made the mistake of tasting a ghost chili pepper. After barely nibbling it, she thought that she was about to explode. Drinking from the Phlegethon was like gulping down a ghost chili smoothie. Her sinuses filled with liquid flame. Her mouth felt like it was being deep-fried. Her eyes shed boiling tears, and every pore on her face popped. Her body folded into itself, gagging and retching, her whole body shaking violently.

The convulsions passed. She took a ragged breath and managed to fix her posture. She felt horribly weak and nauseous, but her next breath came more easily. The blisters on her arms were starting to fade. She slowly put on her hoodie so she couldn't see the blisters or the goosebumps on her arms.

"Ugh," Percy said, distracting Mia from the shitty feelings she felt from the fire water. "Spicy, yet disgusting."

Annabeth laughed weakly. "Yeah. That pretty much sums it up."

Percy turned to Mia. "You saved us."

"For now," she said. "The problem is, we're still in Tartarus."

Percy blinked. He looked around as if just coming to terms with where they were. "Holy Hera. I never thought . . . well, I'm not sure what I thought. Maybe that Tartarus was empty space, a pit with no bottom. But this is a real place."

Mia recalled the landscape she'd seen while they fell — a series of plateaus leading ever downward into the gloom. She couldn't believe it, either, that they were here.

"We haven't seen all of it," Annabeth warned. "This could be just the first tiny part of the abyss, like the front steps."

"The welcome mat," Mia muttered.

They both gazed up at the blood-colored clouds swirling in the gray haze. No way would they have the strength to climb back up that cliff, even if they wanted to. Now there were only two choices: downriver or upriver, skirting the banks of the Phlegethon.

"We'll find the way out," Mia said. "The Doors of Death."

"We have to," Percy agreed. "Not just for us. For everybody we love. The Doors have to be closed on both sides, or the monsters will just keep coming through. Gaea's forces will overrun the world."

And you've always been worth it to me, Mia.

She had to survive. Not for herself, but for everyone else — for Nico, Hazel, and the Starfury corporation.

"Well." Annabeth signed. "If we stay close to the river, we'll have a way to heal ourselves. If we go downstream—"

Percy's eyes locked on something behind Annabeth. Mia glanced over as a massive dark shape hurtled down at Annabeth — a snarling, monstrous blob with spindly barbed legs and glinting eyes. Annabeth was frozen, staring at Arachne in shock.

Mia heard the familiar SHINK of Percy's ballpoint pen transforming into a sword. His blade swept over her head in a glowing bronze arc. A horrible wail echoed through the canyon.

Mia sat there, stunned, as yellow dust — the remains of Arachne — rained around her like tree pollen.

"You two okay?" Percy scanned the cliffs and boulders, alert for more monsters, but nothing else appeared. The golden dust of the spider settled on the obsidian rocks.

Mia stared at him in amazement. Riptide's Celestial bronze blade glowed even brighter in the gloom of Tartarus. As it passed through the thick hot air, it made a defiant hiss like a riled snake.

"She . . . she would've killed me," Annabeth stammered.

Percy kicked the dust on the rocks, his expression grim and dissatisfied. "She died too easily, considering how much torture she put you through. She deserved worse."

"Then I should've killed her," Mia grumbled. "This is why you get a Stygian Iron weapon."

Naturally, Annabeth ignored her. "How did you move so fast?"

Percy shrugged. "Gotta watch each other's backs, right? Now, you were saying . . . downstream?"

Annabeth nodded. The yellow dust dissipated on the rocky shore, turning to steam. At least now they knew monsters could be killed in Tartarus . . . though Mia had no idea how long Arachne would remain dead. She didn't plan on staying long enough to find out.

"Yeah, downstream," Annabeth said weakly. "If the river comes from the upper levels of the Underworld, it should flow deeper into Tartarus—"

"So it leads into more dangerous territory," Percy finished. "Which is probably where the Doors are. Lucky us."