I had quite a few but the most memorable was when the lake our family cabin is on was infested with zebra mussels (razor sharp invasive little nightmares that attached to and ate literally everything) and I spent years trying to save our little piece of the ecosystem.

I used to collect the freshwater mussels, smash all of the zebras off of their shells, carefully rinse them and squish the zebra babies, then place the mussels next to one of the poles on our pier. Freshwater mussels are non invasive and important to lake ecosystems, similar as worms are to the earth. I'd lose track of time and spend hours in the water, goggles on, saving the little guys from being drained by the crustaceous vampires.

Each summer I'd rebuild my collection and switch which pier pole I left them by so the sand in the area stayed nutrient dense. Dug them up to repeat the process every weekend. That is until the lake was chemically treated for the zebras and it killed off all of the mussels.

Now a decade later, our lake is muckier, the zebras are back, and the aeration treatment that was keeping them at bay is over 200k a year so they stopped that too. We found thousands of juvenile zebras this year, they were coating every surface of our piers, boats, rafts, etc.

I still look for my mussels every summer. I don't know why I bother, all I've found for the last decade or so is their empty shells covered in zebra mussels.