But deposits large and concentrated enough to be worth mining are indeed rare.
但是，沉积或富集到有开采价值的却实在难找。
2
Such concentrated deposits ought to make underwater mining highly efficient.
这样的集中式分布可以使水下开采效率大大提高。
3
Instead, Chinese Banks have focused on buying small stakes in multinationals while resources companies have concentrated on individual mining projects.