It's a fantasy role-playing game.

DM: You are Vex Vex'ahlia, a half-elf ranger . You are the member of the famous group of adventurers. Previously, Before it flies away, the duck reveals that there are two other entrances into the mountain. Meanwhile, you has slipped away to follow the footsteps around the mountain.
DM: You follow the tracks, and about 200 or so feet around the edge, and you have to follow them between a few smaller rocks that are jutting out at the base of the mountain, and the terrain itself rolls a bit, and cracks in places. There are small fissures from recent rains that caused the earth to well up and tear in places, and eventually the tracks curve in towards the mountain and end, and it's just sheer mountain. There's no sign of an entryway visually.
Player: Can I search for one?
DM: Make an investigation check.
Player: Do I get advantage because it's a mountain?
DM: You do, actually, yes. Favored terrain.
Player: Fucking balls. Perception?
DM: Investigation.
Player: No, I want perception.
DM: You're getting investigation.
Player: 11.
DM: 11? You have no idea where the entrance is. The tracks end in this general vicinity, and it's just rock. It doesn't even look like there's handholds.
Player: Weird. I wonder if it's an illusion. Can I put my hand on the rocks and see if there's an illusion?
DM: Okay. You put your hands out to feel against the rock, and your hand passes through.
Player: What-what! I feel like Vex is really good at finding illusions. Oh, shit! Okay, well, can I put the front of my face through the wall and see what I see?
DM: Sure. You poke into the darkness, and it is pitch-black on the inside. As you poke past the illusion, it's about two inches or so of this rocky, heavy orange-red stone, and as you peer in you can see a cavern about ten feet wide or so and about 12 to ten feet tall. Natural along the walls, you can see the ground itself is smoothed over, and it's straight. It's a straight tunnel. It's not a rocky natural formation, it's like it was pushed and carved into the rock, like something heavy or something very focused drove through. You glance over to the side, and you catch what appears to be a face in the wall, about that big around. It's featureless, you just see two eye sockets, a mouth, and a nose. You see a flash of green light. I need you to make a constitution saving throw.
