We perceive just a tiny proportion of the phenomena that surround us everyday. Sensory Threads is a technologically mediated collective sensing expedition which allows people to explore imperceptible phenomena in the world around them. We present a description of the technical system and reflect on our design decisions. Key questions we are concerned with centre around how to represent imperceptible phenomena in real time both inside a building, and outside, and how to engender a sense of collective engagement. We also consider the value created through the multi-disciplinary work involved in constructing such an experience.