[etangent]

Of course they are artifacts -- everything in that picture is an artifact, by definition of how it is produced. But these artifacts are produced under specific conditions, which presents a certain window into the structure of what is being visualized. t-SNE is a different, older, method compared to UMAP, and it over-emphasizes local structure at the expense of global structure.

Example of what could cause a swirly chain in UMAP: if A related to B and B relates to C but A does not relate to C and so on. IMO that's a valid structure to visualize as a swirly chain. If re-run multiple times, of course you will get that chain in different locations and so on. But it is interesting that it is there.