Cumulonimbus Cloud.
This was taken late in the evening from my back garden. I watched this stormcloud develop from nothing in a matter of a few minutes. With the evening sun reflecting from it it was quite a spectacle. I emailed it to the meteorological office in Norwich and was told it was quite a catch as there were only three of these in our region on that evening. It's a bit fuzzy as I enlarged it quite a bit.