Rising sea levels, fueled by polar ice melt and thermal expansion, pose an urgent global threat. Already surpassing 20 cm since 1880, sea-level rise accelerates flooding, coastal erosion, and saltwater intrusion, endangering hundreds of millions in low-lying regions. Vulnerable island nations and coastal megacities—from the Maldives to Venice—face submergence, expensive infrastructure damage, and economic instability. Coastal ecosystems like mangroves and coral reefs, crucial natural defenses, also suffer from rising seas. Proposed responses include aggressive emissions cuts, protective engineering, and nature-based solutions such as wetland restoration. For some areas, “managed retreat” may be inevitable. Immediate action, cooperation, and long-term planning are essential.