On a sunny July afternoon along the shores of Hong Kong’s Lamma Island, the receding tide reveals an unassuming patch of greenery that scientists describe as the “ultimate underwater superheroes” for boosting marine biodiversity. Seagrass – an easily overlooked marine plant – can lock away climate-warming carbon 35 times faster than tropical rainforests. But levels of this vital vegetation in local waters have plummeted by 60 per cent over the past two decades, while sightings of local green...