Below the surface of Scotland's urban waterways, enthusiasts are plunging magnets into the dark depths, unearthing metallic detritus discarded decades ago, or yesterday. Nalini Chetty follows magnet fishers Calum Black, Graeme McShane, John Robertson and James Pearson of Edinburgh Magnet Fishing on a day out down the canal, wondering why they do it and what they might find. Will it be treasure, or shopping trolleys?