I had never heard of the viola organista, an instrument concocted in the imagination of Leonardo da Vinci, until I saw a feature on it in the online periodical 3 Quarks Daily. Not a reality in da Vinci's time, the part piano-part cello was built by Polish concert pianist Slawomir Zubrzycki. This video shows Zubrzycki in his first performance on the unusual instrument this past October:
According to various published articles, it took Zubrzycki three years and 5,000 hours to build the instrument. The sound is more than satisfying.
"Polish Concert Pianist Bilds a 'Viola Organista Based on a 500-Year-Old Leonardo da Vinci Sketch", This Is Colossal, November 18, 2013 (This article includes an image of da Vinci's design and a close-up of what Zubrzycki built. Additional closeups may be seen on this Polish blog.)
"Leonardo da Vinci's wacky piano is heard for the first time, after 500 years", The Age, November 18, 2013