A group of kids canoeing in Canada’s Killarney Provincial Park are paddling across a serene and unnaturally turquoise lake. It’s a hot sunny day, and a thirsty boy dips an aluminium cooking pot into the water to refill his fellow canoeist’s canteens. In a momentary lapse of concentration, the pot slips from his grasp. As it sinks underwater beyond reach, what’s incredible is that it’s visible all the way down to the lake floor some 50ft (15.2m) below.

It’s the mid-1980s. One of the kid canoeists is me, and there’s an unfortunate explanation for this water clarity. This lake, near the nickel and copper smelters of the town of Sudbury, Ontario, has been radically altered by acid rain. Almost every living thing in the water – like the tiny algae that would normally block light from reaching the depths – has gone, leaving the water here and in lakes across the region a beautiful but eerily lifeless aquamarine.

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Fast forward to 2019 and another set of lakes in a remote corner of north-west Ontario. Biologist Cyndy Desjardins is sipping coffee at breakfast following a nocturnal boat trip at the International Institute for Sustainable Development’s Experimental Lakes Area (IISD-ELA). Smiling but sleepy, she spent much of the night working in nearly pitch-dark conditions, surveying for tiny monster-like creatures: freshwater opossum shrimp called Mysis relicta. Desjardins is part of a team attempting to close the loop on an acid rain experiment that began in the 1970s.