Very cool concept and fun gameplay. I really like the idea of keeping track of momentum and having fall damage being the main threat in a game, but I feel as though the cues could be a bit more clear. It wasn't immediately obvious that the wind sound effect was tied to high momentum, so for a good chunk of time I was looking back and forth between the percent counter and the character, which isn't that great (it got much better once I was able to focus on just the character). This could probably be avoided with visual cues, things like wind particle effects to go with the sound, or a red border that appears if you're going to fast, or a sprite animation on the character that makes him look scared (since he should be the focus of the player's attention anyway).
A few other minor things, the checkpoints should probably activate when you pass the level they're on, and should absolutely have a visual change when you do activate them. The tutorial might be better if it's a bit more like the main game (maybe it could be themed after the last cave the protagonist explored), and the death barrier needs to be lowered. And I personally think some of the spikes could be made a bit more obvious, they look great but blend in with the background a bit too much when going at high speeds. And this last one is an incredibly minor (and somewhat petty) nitpick, but in my opinion the newspaper that appears on death should replace "found dead" with "presumed dead", since "found dead" implies that other people just casually meandered into this cave that no one's ever successfully explored before.
That said, the graphical style is amazing, the gameplay is very satisfying, and I really like the foundation for the story (maybe it could be extended so that you find the earlier expeditions that the prologue mentioned).
Overall this is a very solid base for a game, and incredibly impressive for something that was made in a week. It's definitely something that could be developed into a larger title.