"Dark layer" is a descriptive term that can refer to a variety of things. When it comes to soil science, a "dark layer" may refer to humus, the dark organic material that accumulates in the upper layer of soil. In geology, a "dark layer" may refer to a stratum or layer of dark-colored rock, such as basalt. In photography or art, a "dark layer" may refer to a layer of dark pigment or ink. Depending on the context, other synonyms for "dark layer" might include "black layer", "shadow layer", "murky layer", "opaque layer", or "gloomy layer." Ultimately, the choice of synonym will depend on the specific context and intended meaning.