Abstract:The article is an exploration of urban imaginaries emerging through a play with materials. Starting from a complex activist exercise for reimagining the space of a park in decay, whose protagonists are children, we propose a reflection on the productivity and resilience of matter. We argue that a new materialist sociology is one that takes disappearances seriously. Capitalism renders space abstract not only through flow and circulation, but also through stillness. We follow the curious disappearances and reappearances of the park in question, tracing the mutations of urban planning, of the juridical domain, and of the everyday use of space. Finally, we analyse the making of a maquette of the park by a group of children and their alliances with activists. The maquette is a political "thing": it leads us away from an urban imaginary populated by discrete objects to an urban imaginary of depth and it reconcretises space. Key words: children's imaginaries, radical social imaginaries, new materialism, children's participation.3 Introduction In his writings Althusser (2006) insisted that materialism is the most difficult problem that we encounter as social thinkers. We here propose a sociology of disappearances and reappearances, as part of a new materialism of the encounter. Things are not static, but they are in constant movement. Things leak out of themselves. Things leak into invisibility. The new materialist sociology we argue for is one that takes disappearances seriously.In the past two decades, a creative place of utterance has emerged in critical urban scholarship: it is a place defined by surpassing a firm opposition between poststructuralism (which has tended to dispense with things) and Marxian phenomenology (which has tended to substitute things for objects, and to argue that we have lost the thingness of things to objects). The productivity and the resilience of matter became the most important materialist story to recuperate (Coole and Frost 2010). Matter returned, but in its processual insistence, as matter that becomes, rather than as matter that is.To this discussion, we add two peculiar questions. How can we tell a story of a disappearance of an urban space (here, it is the space of a park) from urban imaginaries? As we show, capital accumulation turns space invisible. How does space reappear through creative alliances of children and activists around materials? By