Ok now to begin with the tunnel.
Make a new document 540x405 pixels in size.
Now, make a Cylinder, like so:


Stretch the cylinder, by clicking and dragging the dot on the top(center) to the right.
Now go to the Edit menu at the top, and while holding SHIFT, click the Rotate around X axis and drag to the right. Holding shift will make the cylinder rotate in 45 degree steps. Do this untill the cylinder is rotated 90 degrees.
Then lower is using Page Down, untill it's halfway in the ground, like this:


Now make the cylinder a lot bigger, like more tunnel-proportions.
If it moves upwards, sink it in the ground till the middle.
Right, now you have a big cylinder sunk into the ground. So whats the tunnel trick?
We're gonna do that right now.
Move the camera INSIDE the cylinder, you'll see a vague light in the Nanoprevieuw, like so:


Make a Radial Light, and move it with the arrows till it's about above the camera.
Duplicate that light (Ctrl+D) and move it forward about 20 steps (1 step = pressing an arrowkey 1 time). The result should look a bit like this:


Now for the textures. I chose to make a water tunnel, so i chose for the Caribbean Resort under Waters & Liquids, and the Wet Rock for the Cylinder under Rocks & Stones.
Final result on the tunnel:


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