Is it an old blanket? Is it an old whale placenta? Is it an old whale placenta wrapped in an old blanket? It's actually an "enigmatic deep-sea jellyfish" aptly dubbed Deepstaria enigmatica.
First described in 1967 by prominent marine biologist Sir Frederick Stratten Russell, the jellyfish was particularly notable for not resembling a jellyfish.
From Russell's description:
On opposite sides of the umbrella are two large tubular shaped processes…It has a yellowish brown tinge…The radial canal system is most striking. It consists of a meshwork, likened by Dr. Barham to wire-netting.
There is a video of the scyphomedusa Deepstaria enigmatica "floating" around right now. This bag-like jelly is not that rare, but is large, so rarely seen intact. In the video, the swirling from the sub makes the medusa appear to undulate and it even turns inside-out. Here are some images of a more "typical" specimen. The web-like pattern is not a nerve net, as some comments have said. It is branches from their digestive system.
The more you know!