Seaweed

These are marine algae, not plants and definitely not weeds. Though we could call them sea vegetables, we’re not going to. They were undoubtedly among the first foods consumed by humans, are still harvested off the coasts of almost every continent, and are (obviously) natural and incredibly nutritious—high in protein, vitamins (even B₁₂, rarely found outside of meat), minerals, and trace elements (rapidly disappearing from land vegetables due to soil depletion).