Proof that The Fire River is MADE OF FIRE

"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."

- House of Hades, Chapter VII (Annabeth), Pg. 40

"Annabeth 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—”'"

- House of Hades, Chapter VII (Annabeth), Pg. 41

"On first contact, the fire wasn’t painful. It felt cold, which probably meant it was so hot it was overloading Annabeth’s nerves. Before she could change her mind, she cupped the fiery liquid in her palms and raised it to her mouth. She expected a taste like gasoline. It was so much worse. Once, at a restaurant back in San Francisco, she’d made the mistake of tasting a ghost chili pepper that came with a plate of Indian food. After barely nibbling it, she thought her respiratory system was going to implode. 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. She collapsed, gagging and retching,
her whole body shaking violently."

- House of Hades, Chapter VII (Annabeth), Pg. 41

"Desperately, she cupped more fire in her palm. Ignoring the pain, she dripped the liquid into Percy’s mouth. He didn’t respond. She tried again, pouring a whole handful down his throat. This time he spluttered and coughed. Annabeth held him as he trembled, the magical fire coursing through his system. His fever disappeared. His boils faded. He managed to sit up and smack his lips."

- House of Hades, Chapter VII (Annabeth), Pg. 42