n.

1. 下層；下層土壤；底層
   Divers found this unique species of fish at the substratum of the sea.

   潛水夫在海的底層發現了這個獨特的魚種。

2. 基礎

n.

1. an underlying layer or substance, in particular a layer of rock or soil beneath the surface of the ground.
2. a foundation or basis.