Thanks for trying, sadly, it's not one of the Earth series. I saw the cover art for Earth 2160 with a planet in top left similar to my photo, but the main menu is different.My guess is one of TopWare's Earth Series. My understanding is that the Eurasian Dynasty faction has underground tunnels for some of their logistics. Unfortunately I don't play a lot of RTS, so this is only a guess.
The only other game I remember with a similar system is Petroglyph Games' Grey Goo, but that was released in 2015...
I checked in the past hundreds of games from multiple lists like MobyGames, UVL, IGN, Wikipedia etc. in the 1997-2007 range.
I guess it must be an obscure game not listed on those lists or further search had incomplete information (screenshots, gameplay videos) to identify it correctly.
Another scenario considered was if it was the unlikely and rare case when a demo is available before the full release, but the game is cancelled later. Unseen64 has this type of list, no match found though.
This has become for me the most mysterious RTS game in the world.