The bulk of clandestine operations are related to the gathering of intelligence, typically by both people (clandestine human intelligence) and by hidden sensors. Placement of underwater or land-based communications cable taps, cameras, microphones, traffic sensors, monitors such as sniffers, and similar systems require that the mission go undetected and unsuspected. Clandestine sensors may also be on unmanned underwater vehicles, reconnaissance (spy) satellites (such as Misty), low-observability unmanned aerial vehicles (UAV), or unmanned detectors (as in Operation Igloo White and its successors), or hand-placed by clandestine human operations.