The Barragem de Alqueva/Embalse de Alqueva (Relation Number 1.464.895):
With a surface of 250 km2, a lakeside of 1.200 km (about 1.000 km on Portuguese territory, the rest in Spain) and a length of about 85 km the "Barragem de Alqueva" is today the biggest artificial lake in Europe.
It would have been impossible to map this object without the help of aerial imagery in the detail that BING gives us. The only other way to have mapped this for OSM would have been by using official data from some other source. It took 4 months to complete.
I do not know how many nodes and ways (and islets) it contains, as I cannot download it in JOSM as a single entity as it is too large. I think I mapped about 100-200 islets and over 10.000 nodes but it is just a guess.
View it in OSM:
Website in Portugies, German, English