In the Festspielhaus in Bayreuth, the audience cannot see the orchestra. The pit is covered. It is called the mystischer Abgrund, the mystic abyss. Among opera houses, the hidden pit is unique to Bayreuth. The festival was first held in 1876.
The festival’s own magazine explains why. Richard Wagner wanted the Mühe der Tonerzeugung, the effort of producing the sound, to remain invisible. The musicians were not to be seen by the audience, and no light from the pit was to reach the auditorium. The listener should fall under the spell of the music ohne jegliche Ablenkung, without any distraction.