The sphenoid sinus is one of the four pairs of paranasal sinuses located within the skull. It is situated behind the nose and between the eyes, specifically in the sphenoid bone. These air-filled cavities help to lighten the weight of the skull, improve voice resonance, and provide insulation for surrounding structures. The sphenoid sinus is shaped like a butterfly and varies in size and shape among individuals. It drains into the nasal cavity through an opening called the sphenoethmoidal recess. Infections or blockages in this sinus can lead to conditions such as sinusitis, causing symptoms like headache and facial pain.