A 43-year-old male scuba diver presented with an acute history of painful unilateral visual loss after scuba diving. He had right-sided retrobulbar pain and headache. He was known to have sinusitis and had transient visual loss in two previous episodes after scuba diving. His visual acuity was hand motions and 20/20 in the right and left eye, respectively. There was no proptosis. He had a right relative afferent pupillary defect. Colour vision was normal in the left eye and absent in the right eye. Fundal examination revealed healthy discs and macula bilaterally. He was assessed as a right optic neuropathy, possibly secondary to compression. An MRI of the brain revealed a large sphenoidal mucocele extending into the right optic foramen. He was treated with oral steroids, antibiotics and nasal decongestants. He underwent endoscopic intranasal sphenoidectomy and marsupialisation with return of his visual acuity to 20/25 in that eye.