Areas that are naturally at exactly sea-level are always coloured blue regardless, that's why this map of England's fen-land is peppered with blue, even at + 0m: fenland at + 0m.

当然低于海平面的地区总是蓝色，这就是为什么即使是在+ 0m的时候英格兰的沼泽地也会有零星的蓝点。