In 1994, researchers in California and Arizona turned discarded tyres into underwater reefs and found 20 times more fish in concrete canals

• When engineers lined canals across the dry American Southwest with concrete, they solved one problem while creating another.
• The hard surfaces helped stop precious water from disappearing into the soil, but they also stripped away the irregular edges, vegetation and sheltered spaces that aquatic life had used in the old earthen channels.
• Fish eventually returned, yet their numbers, diversity and biomass remained lower.