n.

1. 下层；下层土壤；底层
   Divers found this unique species of fish at the substratum of the sea.

   潜水员在海底层发现了这个独特的鱼种。

2. 基础

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.