Time Out says
The controversial tower was designed by French arquitect Jean Nouvel and is owned by the water company, Aigües de Barcelona. Nouvel was inspired by Gaudí's architecture and the mountains of Montserrat to create this impressive building that towers at 142m (465.8ft) over the Poblenou neighbourhood and changed the Barcelona skyline.
It was built following bioclimatic architecture standards which take advantage of the climate and environmental conditions to reduce energy consumption and improve quality of life for those who work in the building.
A main attraction is the changing lights. Check the website for dates of special light patterns throughout the year.