How do I enter a tunnel?

[Murphy]

I have discovered tunnels on planets but I cannot enter any of them.
The game doesn't prompt "enter? (y/n)" when I stand on the entrance. Pressing the portal key does nothing. I also tried to remap the key, but no dice.

Do I need some kind of item to enter, or what?

[Spacehamster]

Good day, Murphy.

Well, you don't. Because tunnel is not anything like cave\ladder\etc. It just a big boring straight hole through mountain what allows you to pass through such usually impassable terrain. That why very big guns and drills very helpful (albeit rarely) during exploration of some planets.

[Murphy]

Huh. So they're like, horizontal tunnels through the mountains?
The wiki seems to disagree. There's even a page named Tunnels. Though what's described there just doesn't happen in my game. I've never found a tunnel that I could enter.

[Murphy]

Okay I figured it out.
There are two kinds of tunnels, both are called "natural tunnel". One is a stairs down, and has a different tile from the horizontal tunnel.

[Spacehamster]