Mining two tiles revealed a tunnel?

[Gensokyo]

(I tried searching for this but with the keywords mining + reveal, cave and opening didn't show any results so my apologies if this has already been answered many times)
Upon my search for random ores I had stumbled upon some natural coal in the walls, as this was good for my smelting because I just started this settlement, I decided to start digging into it straight away.

To my surprise after breaking two tiles of mine blocks, the cave by itself decided to carve out this collection of tetris blocks into itself, as seen that it isn't hand dug by the radius of the support. Generally I would shrug it off, but my friends have instilled paranoia into me by constantly telling me that there's going to be a troll at the end of that trail.

Is this just a natural thing that happens, or am I cursed with a big green fellow at the end of the tunnel road, safeguarding his delicious q17 coal?
Thanks.

[Ysh]

(I tried searching for this but with the keywords mining + reveal, cave and opening didn't show any results so my apologies if this has already been answered many times)
Upon my search for random ores I had stumbled upon some natural coal in the walls, as this was good for my smelting because I just started this settlement, I decided to start digging into it straight away.

To my surprise after breaking two tiles of mine blocks, the cave by itself decided to carve out this collection of tetris blocks into itself, as seen that it isn't hand dug by the radius of the support. Generally I would shrug it off, but my friends have instilled paranoia into me by constantly telling me that there's going to be a troll at the end of that trail.

Is this just a natural thing that happens, or am I cursed with a big green fellow at the end of the tunnel road, safeguarding his delicious q17 coal?
Thanks.

It is call a ''gallery,'' seeing here for some information.

[Gensokyo]

It is call a ''gallery,'' seeing here for some information.

Ah, thank you.