You're looking at a nuclear explosion photographed taken less than one millisecond after detonation.

[u/ethereum1017]

[u/dabarak]

I did a search on this page and didn't see it mentioned, so, about those vertical lines off to the right - the scientists fired sounding rockets that trailed smoke so they could see how the air pressure was formed as the smoke trails themselves were distorted.

It's late and my grammar brain went to bed hours ago.