Great game. Should have an upgrade that gives you a jetpack or mini helicopter to get you out of the very vertical tunnel you may make. Also should make the day after clicking end day because getting stuck underground and unable to exit despite having a recharge from the solar powered battery is annoying.
The portal thing is a little annoying. Sometimes I'm so caught up in solving the puzzles that I forget which portal was the one I last placed. So could you change it to the Portal style way where the left mouse button places one portal while the right mouse button changes the other?