This guy collected a gallon of seawater, some sand, and seaweed, poured it in a jar, and sealed the lid. He inadvertently collected a lot of tiny animals, too, and a year later the jar is a vibrant closed ecosystem, though it seems to be in a bit of decline due to a massive heat wave.
From the YouTube description:
A year ago I made this huge natural native saltwater ecosphere in a jar. It has had a lot of ups and downs, but to this day is still very successful. The ecosphere has housed crabs, starfish and a lot more and is currently still housing a lot of crustaceans, paramecium, worms, other invertebrates and even spionid worms.
Image: YouTube/Life in Jars