Very clever mechanic! I like how it's all about deciding between which obstacles you're immune too and how fast you go through these tunnels. Also I'm fond of the pixel-styled graphics. I had a lot of fun playing many rounds to try to go further and further (some depth counter or so would be a nice add indeed).
There is also room for smoothing a bit the movement and adding some animations. Currently, moving the player horizontally is instant: you could add some inertia to this (just a tad so that it doesn't raise difficulty too much). And the player and obstacles could benefit from a basic animation (even a 2-frame one).
Still a very clean game overall, congratulations!