For about six straight hours after taking magic mushrooms – psilocybin – I had visions of a vast, skeletal shark coming at me out of the watery gloom, mouth open, teeth inches from my face. It wasn’t a hallucination – I only saw the shark when my eyes were shut – but even with my eyes stretched wide I felt dread, the same blank terror I had felt the year before when in the spirit of happy enquiry I’d taken acid.