This article argues for an understanding of material geographies as invested with an unconscious dimension. I put forward the notion of spacetimeunconscious not as an inverse, double, or ‘other’ to Karen Barad's concept of spacetimematter, but as a supplement. Manifesting the spacetimeunconscious through the technique of montage, I draw together a range of phenomena, from the icing of water and the flashing of lightning to the awakenings of traumatised and displaced subjects. Across these juxtaposed parts, I argue that the unfolding of space and time responds to the enigmatic, irreducible message of the unconscious in the real. Spacetimeunconscious arrives as the ambassador of an unknown knowledge remembered for – or in the place of – a forgetful substance: water, dreamer, or electron. In an echo of the analytic method, I use montage to create generative connections, discontinuities, and instabilities between events, poetry, literature, and film, in the interstices of which the spacetimeunconscious may make an appearance.