There are many Symphonies No. 10 by Gustav Mahler, or none. The situation is rare, if not unique, in the history of music. Basic facts: Mahler finished the Ninth Symphony and Das Lied von der Erde in the summer of 1910. At the same time he discovered that his wife Alma was having an affair with Gropius, and that he had an incurable heart complaint and hadn’t long to live.