Lightning-related accidents were relatively frequent among age-of-sail seafarers, due to tall ships’ exposure and prominence above the sea. Yet lightning remains largely neglected in the historiography on sailors’ experiences. Examining a range of contemporary medical and philosophical literature, shipboard surgeons’ journals, operational correspondence and seamen’s memoirs, this article argues that lightning strikes created unique moments of epistemological and social crisis aboard naval ships. With its awesome multi-sensorial manifestations, divine symbolism and catastrophic and erratic effects on human bodies, lightning loomed large in many seafarers’ consciousness, as a powerful source of panic and trauma. At the same time, despite contemporary developments in natural philosophical understandings of electricity, surgeons’ training on how to treat these injuries remained limited. As a result, lightning could substantially affect naval shipboard relations between officers and ‘common’ seamen, creating challenges to the former’s authority and mobilising a range of competing emotions and knowledges. Considering lightning illustrates the fruitfulness of looking at uncommon but devastating types of injury, for historians interested in medical authority and in the doctor–patient relationship. It also helps us to start sketching accounts of seafaring bodies, health and maritime medical and religious cultures that give the sky its due place alongside the water.