A toxic border river is putting lives and livelihoods at risk in northern Thailand. The Kok River, once rich in fish, is now polluted with arsenic, mercury, and other hazardous contaminants. Fishers and farmers report dying fish, crop failures, skin diseases, and growing fears for their health. Scientists warn that contamination levels in the river exceed safety limits. Evidence points to rare earth mining operations across the border in Myanmar, where extraction has rapidly expanded. But the issue is politically sensitive and remains unresolved, with authorities slow to act. As pollution continues to flow downstream, concern is rising that the damage could spread beyond northern Thailand, threatening ecosystems and communities across the wider Mekong region.