Winnipeg Free Press: More than 900 metres beneath the surface of the Gulf of Mexico is a steep sinkhole inhabited by tulip-shaped sponges, neon fish and rarely-seen crustaceans, skittering along a sunless ocean floor. Called the Tortugas Valley, it’s a world already threatened by human activity, writes Alanna Mitchell, even though just a handful of people, herself included, have ever glimpsed this underwater treasure. The award-winning Toronto author and environmental journalist was more than a …