What's the difference between 地下 and 地中?

Question

I recently learned two different kanji, both tied to the idea of something being below the ground:

• 地{ち}下{か}: defined in WaniKani as underground, as well as basement and cellar; also defined as underground, as well as below the ground in the dicitonary I checked.
• 地{ち}中{ちゅう}: defined in WaniKani as underground, as well as subterranean; same definitions in the dictionary I checked.

They clearly have distinct definitions, and thus presumably usages as it relates to their secondary definitions, but I was wondering if there is any nuance to the primary usage — underground — or if the two terms are completely interchangeable.

Answer 1

From my experience, 地下 means "underground" (as in "below the surface of the earth"), while 地中 basically means "buried".

地下 is much more commonly used, and I believe can really be used pretty much anywhere 地中 could be used as well, but can also be used for some things that 地中 does not apply to.

地中 has a sense of being put in the earth and then covered over, or being surrounded by soil, etc. On the other hand, 地下 can refer to things which are just located in an open space which exists below the ground (for example, caves, or cellars, or subway tunnels, etc).

So, for example, an irrigation pipe might be 地中 (it is buried in the ground, with soil piled in on top of it), but a subway train is not 地中, it is only 地下, because it is not actually in contact with and surrounded by dirt, it just moves through a space which is below surface-level.

Also, as was mentioned, 地下 can be used in a figurative way to refer to illicit or hidden things, similar to how "underground" is used figuratively in English, while 地中 cannot.

Answer 2

Semantically both are mostly the same. When I was googling, there was even a usage like: 地中における地下杭の存在.

I think the difference lies more in usage. When appending directly to another noun to mean something underground, 地下 is more likely to be used.

• 地下鉄 (railway), 地下資源 (resources), 地下水 (water)

In these, 地中 cannot replace 地下. I realized there is a word 地中熱, but this is not very common. In practice, 地中 is most often followed by a particle.

In terms of nuances, 地下 sounds more substantially underground while 地中 just literally means below the ground however shallow it is. For example,

• 地下に潜る can mean figuratively (e.g., for a criminal) to hide, escape; it could be used for a mole as well, but 地中に潜る sounds better for animals or insects.

• It is ok to say 地中に埋まった種 seed in the ground (while 地面に埋まった may be more normal); 地下に埋まった種 sounds odd. At best it could mean what was buried deep underground as a treasure or something.