Mud

You find the entrance to what appears to be a cave in the tall, cool Rocky Mountains in southern Colorado.

Inside, you find a small domed chamber.  In the glimmering twilight, as you walk around on the sloping, muddy floor, you quickly discover that this cave is not very interesting.  There are no exits to deeper places in the cave; it's just a single room.

But wait...

Out of curiosity, you wonder why the mud is sloping to one end of the room.  Getting down on your hands and knees, you start scraping the thick, clay-like mud away from the wall.  Sure enough, there's a passage!

It's tough, sweaty work, but you manage to smear and scrape away enough of the sludge-like mud to make your way into the cramped passage beyond.  What you see is incredible!  Limestone formations hang from the ceiling like icicles, as far as you can see down this sloping passage.

But your wonder is short-lived.  It seems that your scraping and shoving has triggered the mud to start flowing!  It is oozing quickly into the passage behind you, encasing your legs and threatening to knock you over.  You scramble to get on top of it, but the sheer volume of the stuff is making it increasingly difficult to move.  As you watch in horror, the mud starts to fill the hole you made into this passage.  As the twilight fades and winks into darkness, you begin to panic.  The mud is filling this low cave, and it won't be long before it's over your head...

Mud.  It's in Lone Spelunker now.

You can clear it away with your shovel, but be careful.  Do it in the right place, and you might discover a hidden treasure or a blocked passage.  Do it in the wrong place, and you might trigger a mudslide that suffocates you...