Pacific Diver (Pacific Loon) Gavia pacifica
This 'new species' was split from Black-throated Diver Gavia arctica recently. There were no accepted records in the UK, or anywhere in Europe, prior to 2007. However early that year not one but three different individuals were all identified in the UK (first-winters in Yorkshire and Pembrokeshire and an adult in Cornwall). What caused this little influx is unknown, but it's quite possible that the attention drawn to the form by the appearance of the Yorkshire bird contributed to the discovery and identification of the other two. Others have been reported since and perhaps more will be found now the species is better known here, but with a North Pacific distribution there is absolutely no reason to think that this species should ever be more than an extremely rare vagrant to Europe.
The first photos below show the well-watched Yorkshire bird, taken moments before it flew off, never to be seen again. Also shown is a bird from Cornwall, possibly the same returning adult that has been seen in that county for several winters.