Missing the middle: Deep cyanobacteria layers pose a “known unknown” risk to water security

•A disproportionate focus on surface cyanobacteria has neglected subsurface events.•Deep cyanobacteria layers remained an overlooked concern in cyanobacterial research.•Known unknowns—aware of the risk but unaware of the scale of the risk to society.•Scientific advances in deep cyanobacteria layers ecology will enhance cyanobacterial management.