GG5

I'm familiar with "pit mining" but I agree it's misleading.
A contrasting term is not an antonym.

@@ -12,2 +12,2 @@
-<xref type="ant" seq="1644690">露天掘り</xref>
-<xref type="ant" seq="1644690">露天掘り</xref>
+<xref type="see" seq="1644690">露天掘り</xref>
+<xref type="see" seq="1644690">露天掘り</xref>

特集 - ゼロからわかるマイニング
https://www.hitachicm.com/global/ja/corporate/publicity/magazine/2019/vol128/feature/

It's right in the file name (komatsu is a massive manufacturer of mining machinery, like Catarpillar):
https://www.komatsu.jp/jp/ir/library/results/J_01_Undergrond Ming Business.pdf
坑内掘り事業の概要 (transl. mine:An overview of the underground mining industry)

https://prsubmissionsite.com/private-5g-services-industry-4-0/
Mining in India is of two kinds. The first is called open pit mining. This kind of mining does not require you to get into a tunnel. The other kind of mining is called closed-pit mining. This requires you to go underground inside a tunnel.
そして、鉱石を掘る方法には、大きく分けて「露天掘り」と「坑内掘り」がある（右図参照）。

https://kotobank.jp/word/坑内掘り-62965
ブリタニカ国際大百科事典 小項目事典 「坑内掘り」の意味・わかりやすい解説
こうないぼり
underground mining

Generally "underground mining"
https://context.reverso.net/translation/japanese-english/坑内掘り

The old gloss should not be restored.  "pit mining", in general, will be interpreted as "open pit mining" in English, which is literally the opposite of this term.

The term "closed-pit mining" is used, but is comparatively rare, is never used on English wikipedia, and is almost non-existent in news results. Usually when contrasted directly with open-pit mining.  Google gives a made up number of ~6000 hits for closed-pit mining, vs. 1.1 million for "open-pit mining" and 3.8 million for "underground mining".  I don't think it's worth the gloss. 

Out on a limb and call this [ant], not [see=].  See reference remark:
大きく分けて「露天掘り」と「坑内掘り」がある

"Above the surface" and "under the surface" mining. 

If you start typing 坑内掘り in google, it will autocomplete 坑内掘り 露天掘り
If you start typing 露天掘り in google, it will autocomplete 露天掘り 坑内掘り

jmdictdb already has:
坑内掘り炭鉱
1. underground coal mine

@@ -12 +12,2 @@
-<gloss>pit mining</gloss>
+<xref type="ant" seq="1644690">露天掘り</xref>
+<gloss>underground mining</gloss>

Entry created