Like territory, terrain is a term that has been tied to its etymological roots on terra. This paper seeks to release terrain as both concept and practice from the terrestrial through an analysis of the Cold War‐era case study, Sealab II. This little‐studied project, led by the US Navy, sought to establish the feasibility of sustaining life under the sea and in doing so, provides a rich site of analysis through which to explore the notion of terrain that exists in volume, rather than simply on the earth's crust. Within this immersive voluminous framework, the function of the body is also re‐examined as both a site that experiences terrain, but also one that became a terrain of sorts during the Sealab experiments. The paper concludes by suggesting that understandings of terrain within geographical scholarship would be enriched were they to push off from the earth's surfaces and argues that there is a need to re‐think terrain's relational aspects, re‐root it from terra and re‐orientate it towards the body.