"What is the fifth dimension?" asks Lena Komaier-Peeters from East Sussex.
Proving the existence of extra dimensions, beyond our 3D universe, is one of the most exciting and controversial areas in modern physics. Hannah and Adam head to CERN, the scientific cathedral for quantum weirdness, to try and find them.
Theoretical physicist Rakhi Mahbubani explains why we think that dimensions beyond our own might exist. Adam meets Sam Harper, who has spent 14 years hunting for an elusive particle called the 'graviton', which could provide a portal to these extra dimensions.
But if they exist, where have these extra dimensions been hiding? Sean Carroll from Caltech explains various ideas that have been dreamt up by physicists, from minuscule hidden planes to gigantic parallel worlds.
Producer: Michelle Martin Presenters: Hannah Fry, Adam Rutherford.