In this article, I explore the formation of airspace in Britain from 1910 to 1913. The technology of flight challenged the “flat discourse” of nationalized geography, drawing up instead a volumetric space in the sky as airplanes flew from the Continent to England. The drive to control aerial mobility and convert the sky into a sovereign territory was especially pronounced in Britain. But the challenge of creating a sovereign space out of mobile and transparent air was an intricate problem both in legal and practical terms. This article shows how geopolitical interests called for an upward extension of the Island Kingdom, extrapolating its coastal borders into the sky. However, even as Parliament passed the Aerial Navigation Act in 1913, this legal construction of an island in the air could not endure the agency of airplanes. The formation of airspace, I argue, is a history particularly well suited to showing the dynamic processes by which spaces are assembled, enacted and co-produced by mobile technologies.