Recent studies of 'unconscious working memory' have challenged the notion that only visible stimuli can be actively maintained over time. In the present study, we investigated the neural dynamics of subliminal maintenance using multivariate pattern analyses of magnetoencephalography recordings (MEG). Subjects were presented with a masked Gabor patch whose angle had to be briefly memorized. We show with an unprecedented level of precision, that irrelevant sensory features of contrast, frequency and phase are only encoded transiently. Conversely, the relevant feature of angle is encoded and maintained in a distributed and dynamically changing manner throughout the brief retention period. Furthermore, although the visibility of the stimulus correlates with an amplification of late neural codes, we show that unseen stimuli can be partially maintained in the corresponding neural assemblies. Together, these results invalidate several predictions of current neuronal theories of visual awareness and suggest that visual perception relies on a long sequence of neural assemblies that repeatedly recode and maintain task-relevant features at multiple levels of processing, even under unconscious conditions.