I saw this TED talk recently, and it struck me as a great application for Flutter.
This guy is using discarded cell phones to sense the sound of illegal chainsaws in a noisy forest. You basically need to run an FFT (or FHT?) to check for regular tones in the 20-200hz range. The discarded phones are likely free, but it takes a lot of solar cells to power them. It seems like a purpose-built mesh solution would be a more robust and less time-consuming way to deploy this, and fewer solar cells would be needed.
I'd love it if someone built a system with Flutter to save the rainforest!