We propose developing modular aquafarms for large-scale cultivation and regular harvesting of surface-floating seaweeds such as Sargassum in the centres of the ocean deserts: the 5 subtropical gyres that cover 50 % of the planet’s surface. The fields of algae will be fertilized with warmed, nutrient-rich deep water provided continuously by 400 m long vertical pipes based on the Stommel principle, but greatly accelerated by a counterflow. The combined upwelling and downwelling pipes do not require additional energy input once installed and started. Expanding aquafarms have the potential to produce sea food, seaweed raw material as a plastic alternative, also for manufacturing the pipes, and carbon neutral biofuel in bulk. When aquafarms have expanded sufficiently, the excess seaweed harvest will be compressed into bales to halt microbial breakdown and stored in depots on carefully selected sites of the deep-sea abyssal plain for long-term sequestration. Our nature-based, sustainable endeavour will open unused spaces in the ocean’s vast centres thereby taking pressure off the overexploited ocean margins, preserving biodiversity while providing food and climate security.