Bathybius haeckelii is like a big mystery jello that scientists thought was living goo at the bottom of the ocean.
When people wanted to explore the deep, dark parts of the ocean, they used a thing called a dredge to collect samples. One day, scientists found this big blob of ooze that they called Bathybius haeckelii. They thought it was living because it was made up of all kinds of different things, like cells and fibers.
But some other scientists said that it was actually just a bunch of dead stuff that sunk to the bottom of the ocean and got stuck together. They said Bathybius haeckelii wasn't a living thing at all, but just a really big piece of ocean garbage.
After lots of arguments, people finally figured out that Bathybius haeckelii wasn't a living thing, but just a big pile of dead stuff. It was still really interesting to scientists because it helped them understand how things like ocean currents and water pressure affect the ocean floor.