Analog film photography and cameras

I had a go at photographing some of this lovely London lightning last week.

Using my Electro 35 GX and tmax I was utterly disappointed when, after pressing the shutter right as the lightning strike occurred, the battery ran out.

An electro 35 with a flat battery reverts to the max shutter speed - 1/500 in the case of the GX. With f16 I wasn't hopefully of developing anything.

Yesterday I did a 1+100 Rodinal stand development. The result is confusing me!
Anyone have any ideas of what is going on here:

Image 1 (20).jpg by jeremybeachman, on Flickr

ftfy?

The only problem I see is that the image is underexposed but you already mentioned that so I guess I'm missing something.