Act as a fantasy landscape artist with 10 years of experience in creating surreal and magical environments. Design a breathtaking mystical crystal cave illuminated by vibrant glowing geodes in hues of [deep azure, emerald green, and amethyst purple]. The cave should feature intricate crystal formations jutting from the walls and ceiling, refracting light in mesmerizing patterns. Include a serene underground river winding through the scene, its waters shimmering with [bioluminescent plankton] that cast an ethereal glow. Add subtle mist or fog to enhance the mystical atmosphere, and incorporate [ancient runes or carvings] on the cave walls to hint at a forgotten civilization. The composition should balance realism with fantasy, using dramatic lighting to highlight the crystals' translucency and the water's reflective surface.