Retinex, probably oversharpened.
I had picked this for Life (we are still tied to the tides, check your heartbeat) and was thinking some type of B&W conversion or maybe a duotone or something. But eventually I just had to admit it wasn't a very compelling picture. So this is just the Retinex filter from the GIMP. I googled that, but didn't really understand it. Anyhow, it does seem to give the right type of emotional impact to what wasn't a really great picture.
I did play with many B&W conversion options, tone curves, filters, etc.
I have another snapshot (a planned shot this time) for this 100
Snapshots series I think I will use this effect on also. Perhaps a
B&W conversion will work better, but I think this effect is nice.