As we followed Skilaki along the unmarked path, the distant noise grew louder, like a giant vacuum cleaner pressed into my ears. Soon I had to cover them with my hands.

"My dear sibyl, this noise is unbearable!" Professor Bhegad shouted.

Skilaki stopped at a clearing. She crouched, picking up a clot of soil flecked with pine needles, pebbles, and who knew what else. As she held it toward me, kneading it with her hands, it became rubbery and smooth, shrinking to the size of a vitamin pill. "Insert it into your ear," she said. "You'll be much happier."

"It's dirt!" Cass shouted.

"Give it to me!" Aly grabbed the little pellet and popped it into her ear. She dropped to her knees and dug out another clot of dirt. Quickly she repeated what Skilaki had done, massaging it with her fingers until the grains of soil and tiny twigs smoothed out. Then she inserted that one, too. "Whoa. It works. It feels like Styrofoam."

Cass, Professor Bhegad, and I wasted no time plugging our ears.

"Our natural materials," Skilaki explained, "are multipurpose."

I couldn't believe it. The static noise was nearly gone, but Skilaki's voice was loud and clear. All of our voices were clear. Even our footsteps. Only the frequency of the river's sound seemed to be blocked.

Skilaki gestured into the clearing. "Proceed," she said.

As we cautiously stepped forward, the clouds thinned. I could make out the shape of what seemed to be an enormous river stretching into the thick grayness to our right and left. The opposite bank could have been a football field away or a mile-in this strange landscape, it was impossible to tell.

A silent current raged not two feet beneath us. It seemed weightless, a flow of silver streamers in midair, reflecting light and nearly transparent. It splashed against the steep banks and broke into a spray of droplets. I could feel them on my arms, tiny pokes with no sign of wetness at all.

I removed my plug-but only for about a nanosecond. The static noise was unbearable. "That's what's making the sound," I said. "The river water."

"I don't think that's water, Jack," Aly said, her voice unmuffled and clear.

I stepped closer and knelt by the edge of the bank. The river bottom was alive with movement. But not fish or seaweed. Bright images churned upward, bursting through the sand and mud-people, panoramas, views of villages and mountains in intricate, black-and-white detail. Some seemed harmless and dull, but others were impossible to look at. A gutted home, a screaming face, the twisted grille of a truck.

Aly let out a gasp. Or maybe it was me. I turned away, unable to watch any more.

"This is where you proceed on your own until we meet on the other side, which may be awhile," Skilaki announced. "I'd like to say it's been a pleasure, but I barely remember what pleasure feels like."

Cass's face was taut, his eyes wide. "You expect us to swim across that?"

"Unless you can walk on the top," Skilaki said.

"What is it?" Aly asked.

"The River Nostalgikos," Skilaki replied. "The Greeks have one like it, too, of course."

"I mean, what's the stuff at the bottom?" Aly said.

"Memories," Skilaki said. "The river feeds on them. Our guests arrive with sadness and broken dreams. Their thoughts eat at them for an entire lifetime. They may have an image of themselves they cannot live up to. Or hold a grudge. Or pine for a love that can never be. Nostalgikos makes you face your worst memories and realize how fleeting they are. And if you do face them, it takes those memories away, cleanses them completely."

"So . . . they stay at the bottom?" Cass said softly. "Like old Facebook posts?"

"Ah, but only if you give yourself to the river," Skilaki said. "Fight it, and the bad memories will consume you, like all diseases. I have seen it happen. So tragic. So useless."

"That's it?" I said. "You just wade through, drop the memory, and you're free?"

"Not free," Skilaki said. "All good things require a sacrifice."

Cass paled. "Sacrifice? Are we talking body parts?"

Skilaki gave a wet, rattling chuckle and drew forth a yellowing scroll from her pocket. "If we should become separated," she said, "this will help you reach Artemisia's palace."

Cass stared at it with intensity. I could tell he was memorizing it. I pointed to the river marked Photia, close to the center of the map. "Is this one a memory sucker, too?"

"The River Photia protects the palace," Skilaki said. "For those who have passed through Nostalgikos, who come to Artemisia with a true heart, it will allow safe passage. But if it senses intruders, it will destroy them. And you have no idea how difficult it will be for me to explain that to my queen."

"Wait, that's the sacrifice?" Aly said. "We have to approach with a true heart? Are we going to Artemisia or the Brothers Grimm? I mean, how can we be sure our hearts are true?"

"You cannot," Skilaki replied. "Photia will determine that."

"And if it makes a mistake, we're drowned in a flood?" Cass said.

"Photia is not a river of water," Skilaki said, turning to leave. "And neither is Nostalgikos. Remember, all of you must give in to Nostalgikos. Or the process shall not be complete. I shall meet you at the other side. I have a long path to the bridge. If, by some ludicrously unlikely chance, you should arrive first, wait for me."

"Why can't we take the bridge, too?" Cass pleaded.

Skilaki spun so fast a clump of her hair flew off. "If you do not follow the rules, then you will not see Artemisia. You forfeit your promise. And there are consequences to that."

"Like what?" Aly said.

Skilaki turned away. "You will all share the fate of Radamanthus."

CHAPTER TWENTY-SEVEN.

COLD FEET.

CASS STOOD FROZEN at the side of the river, staring downward. "I can't."

"You were the one who wanted to do this," Aly reminded him. "Why the cold feet now?"

The face of a howling wolf rushed up from the sludgy bottom. Its teeth were sharp and bloody. "That's why!" Cass said.

"Those are just images, Cass," I said, putting my hand on his shoulder as if I weren't scared to my bones. Which I was.

"Hey," Aly said. "What would Marco do?"

Cass spun around. "He'd get us to the other side. He'd face down whatever is in there. And he'd do it with a smile."

"So let him be your inspiration," Aly said.

"I don't see either of you jumping in!" Cass shot back. "We can't do this without Marco. We fail without him. He's brave. Competitive. Fearless. All the things we're not."

Marco.

I'd been doing my best to forget about him. But Cass was right. It didn't feel the same. It hadn't, ever since he'd gone over to the Dark Side.

We needed him. Badly.

And for the first time, I was beginning to feel like we had a chance of getting him back.

"He is competitive," I said. "And right now, our side is winning. We have the Loculi. If there's any chance to get Marco to come back, this is it, guys. Make the Massa fail. Gain power. Continue with the mission of the KI. Marco wants to be with a winner."

Professor Bhegad nodded. "Wisely said."

"Okay, so who's going to lead us?" Cass said, looking at me. "You, Jack?"

"You must all lead," Professor Bhegad said. "Marco will follo-o-o-o-ow!"

His voice became a weak shout as he allowed himself to fall into the river. His body jittered like a scarecrow's, his hair waving like cobwebs in a wind and his glasses flying into the river.

I looked at Aly and Cass. There was no going back now.

I slid off the bank next. My legs made contact with the surface. Like water, it slowed my descent. Like water, it gave me a feeling of buoyancy.

But unlike water, the Nostalgikos felt tickly, like feathers. It flowed in bands of liquid silver, churning hundreds of moving images that boiled upward and sank. Some were minuscule and vague, others enormous and lifelike. I screamed and jumped away as a head the size of a medicine ball emerged directly below me. It rolled back, revealing thick eyebrows raised high into a sharply wrinkled forehead. A face emerged, oozing blood from one eye. Its nose was strangely twisted and its mouth wide open in a silent scream, framed by a matted silver-black beard.

I felt Professor Bhegad's hand on my shoulder. Cass and Aly were in the river now, too. Even though we were different heights, we were all chest deep in the not-water. I had no sensation of sinking, but I couldn't feel the bottom under my feet, either.

"I s-s-saw that face," Cass said. "I want to look for the bridge."

I took a deep breath. "These are images, that's all. Memories that belong to other people."

A severe-looking woman rose up from below, her hair tied back into a bun, a hairy mole on her left cheek. She wore a tight-necked tweed jacket and long skirt, and she tapped a yardstick in her hand.

You will not let this frighten you.

I reached out for the yardstick and felt nothing. My hand passed through the image, and the old woman plunged back downward and out of sight. "Harmless," I said. "Now, come on. Let's get to the other side. Swim. Wade. Whatever."

"Okay," Aly said, stepping toward the other side. "Okay . . ."

As I moved with her, I heard a phone ring. The river seemed to dissolve into whiteness before my eyes. Another image rose up, this one so big and encompassing that it blotted out everything else.

Our old cordless phone. Just the way it was, sitting by the desk in the kitchen. The ring jangles my whole body head to toe.

I'm eating mac and cheese and I nearly jump out of my chair. I hope it's Mom.

But Dad gets there first. He's excited, too. At first I'm mad, mad, mad. I wanted to talk. Then I step back and listen. When he says "Hello?" I get all excited again. My legs can't stay still. I'm dancing like a scratchy monkey. Like I have to pee.

And that's what I remember most. The dancing. The way Dad's face changes. The darkness. The words.

The news that tells me what has just happened in a place at the bottom of the world.

What does "crevasse" mean? I am shouting now. Screaming.

WHAT DOES "CREVASSE" MEAN?

I wanted it to go away.

Every part of me, every nerve in my brain, was trying to dull the image, to shove it away, make it disappear.

"Stay with it . . ." Professor Bhegad was holding on to me.

Remember, all of you must give in to Nostalgikos. Or the process shall not be complete.

I had to do it. But the memory was killing me.

NO!.

I had to take a break. Just a moment.

I would do it. I would try again and succeed. Just not now. I needed to gather strength.

Somehow I managed to turn my head, somehow I made myself stop seeing the phone, the kitchen, Dad's eyes.

Amazingly, the professor had found his glasses, but they were slipping down his nose. His face was twisted into a pained grimace. Next to him, Aly breathed hard, talking to herself, her eyes buggy and white. Cass was far ahead. I couldn't tell if he was crying or laughing. But I could only pay attention briefly. My own pain tore at my insides like wild horses.

Ahead of us, Cass let out a shriek that shocked me out of my own nightmare.

"Cass . . . Cass, my son, let it go!" Bhegad was releasing me, trying to wade toward Cass.

He was twisting side to side, chest high in the tinselly stream, his arms raised above the surface. "Let go, let go, let go, go, go!"

"It's an image!" Aly cried out. "Release it! Do not fight it, Cass!"

"I can't!" he said. "Get it away! Get it away!"

His teeth were bared now, his eyes enormous. Below him, a blotch of red slithered slowly toward him, taking shape along the river bottom.

I grabbed Cass by one arm and Aly took the other, but his eyes were fixed on the river's surface. "No . . ." he said. "Not you . . ."

"Jack, look!" Aly cried out.

A pair of yellow eyes burst through the surface, followed by a snout filled with knifelike teeth, and a pair of leathery wings that seemed to suck the stagnant air from around us.

A blast of putrid breath nearly knocked me off my feet.

The griffin was back.