Prism Waverly wanted to paint a mural the size of a city. She wanted it big enough to swallow a whole cliff, bright enough to see from the surface, and permanent enough that no storm could scrub it away. She had tagged every wall, bridge, and storm drain she could reach. Each piece looked perfect for one tide. Then the water came and licked it blank. Four years of photos filled her archive, and not one painting still existed outside her camera. She tested another wall that morning. She mixed teal ink and threw it hard. The splatter bloomed across the stone, fluid and wide, and she grinned. By the time she circled back, the current had already thinned its edges. The color ran down into the sand like it had never been there. "Stop," said a voice behind her. "You are wasting pigment." Prism turned. A mermaid with diamond skin hovered there, her tail holding every color at once. Prism knew her by reputation. Chakra Crystallis ruled the deep reefs. Her skin sang when the water pressure shifted, and she logged every pulse. "These walls get scoured flat," Prism said. "I need one the tide can't reach." "The tide reaches every surface you know." Chakra pressed a palm to the stone. Her skin trembled. "Four point one hertz. This rock is porous. It will never hold ink." She said it with no sorrow, like reading a number off a dial. Prism's throat tightened. "Then I'm finished before I start." "No." Chakra's chest pulsed once. "There is a wall the tide does not touch. Below the thermal shelf. The heat holds the current off the stone. I have felt it. Granite. Dense. One hundred feet across." Prism stared at her. "You've seen it?" "I have felt it. That is better than seeing." Chakra turned and swam down into the dark water. Prism followed. They dropped past the reef edge, past colder layers, until warm threads touched Prism's scales. A wide granite face rose before her. Blue flames burned in a basin nearby, their light steady on the rock. No sand ground at this wall. No tide pulled at it. The stone was dry of color and waiting. Prism pressed her hand flat against it. It did not crumble. It did not weep ink back at her. For the first time, a surface held still under her palm. "This one keeps it," she whispered. "It keeps it," Chakra agreed. "But you are not alone down here." She pointed past the basin. Prism looked. A blue mermaid clung to the far edge of the granite, her jeweled tail dim at the tip. Six gems had gone dark. She watched Prism with hard green eyes. "That cold you feel under the heat," the blue mermaid said. "It is climbing. It took two tails already. If it reaches this wall, your paint freezes before it dries." Prism had finally found her canvas. Now something underwater was coming to take it.