The night the frost started creeping across the inside of their bedroom window, Thomas stopped pretending their house could save them.

The heat inside the well was suffocating, but the cold radiating from the blue ice on the heart was so intense that Thomas’s fingers went numb the moment he reached over the iron lip.

He climbed onto the edge of the well, his boots slipping on the frozen metal, and balanced himself between two of the massive supporting chains. The stone heart hung directly in front of him, its erratic, crimson pulse warm against his face, while the blue ice on its side sent a shivering, bone-deep ache through his chest.

"Be careful, Thomas," Nicholas called out from the floor, his large hands gripping the iron rim of the well to steady it. "One bad strike and the whole thing might go."

Thomas didn't answer. He set the tip of his cold chisel against the thickest part of the blue ice, right where it joined the metal link of the lowest chain.

He raised his hammer.

Clang.

The sound of the strike was deafening in the confined space of the vault, a sharp, metallic ring that echoed down the vents.

The blue ice did not shatter. It didn't even chip. Instead, a hairline fracture, glowing with that pale, predatory blue light, raced up the chisel, instantly turning the steel tool so cold that it stuck to Thomas’s leather gloves with a sickening hiss of tearing fabric.

Thomas gasped, his grip slipping, but he forced himself to hold on. He raised the hammer again, his muscles screaming under the weight of the heavy iron.

Clang.

This time, a tiny piece of the blue crystal broke away. It fell into the dark shaft below, its blue light fading into the blackness like a dying star.

But as the piece broke, the stone heart let out a violent, shuddering groan.

A wave of intense, dry heat erupted from the core, so hot it singed Thomas’s eyebrows and blistered the skin on his cheeks. At the same instant, a counter-wave of absolute, freezing cold blasted from the blue ice, clashing with the heat in a violent, invisible collision that threw Thomas backward off the well.

He hit the stone floor hard, the wind knocked out of his lungs, his hammer clattering across the floor.

"Thomas!" Arthur yelled, rushing forward to help him up.

The chamber was shaking now. The low, rhythmic heartbeat of the earth had turned into a frantic, chaotic hammering, the vibrations so strong that small pieces of mortar began to rain down from the basalt ceiling.

Above them, through the open hatch, they could hear the terrified screams of the townspeople as the kiln’s floor began to tilt.

"We have to get out of here!" Arthur panic-struck, looked up at the ladder. "The roof is going to collapse!"

"No!" Thomas gasped, dragging himself to his feet, his hand clutching his bruised ribs. "Look at the core!"

The two strikes had done something.

The blue ice on the lowest chain had cracked, but the crack had not stayed in the ice. It had traveled into the black basaltic stone of the heart itself. A deep, glowing red fissure had opened along the flank of the core, and from within the crack, a liquid, molten light—bright as gold—was beginning to seep out, dripping into the well like blood.

Wherever the molten light touched the blue ice, the crystals did not grow. They dissolved, turning into a thick, gray steam that smelled of old sulfur and fresh pine.

"It’s not a flaw," Thomas whispered, his eyes wide as he watched the gold light eat away the blue frost. "The heat... the real heat isn't on the surface. It’s inside. We don't need to thaw the ice from the outside, Nicholas. We need to let the heart bleed."