Along with Ulysses Tholus, it is almost midway. In the middle of the volcano is a caldera, named Biblis Patera, believed to have formed as the result of collapse of the magma chamber during eruptions of the volcano. The caldera is 53 kilometers in depth. (wikipedia entry)
A tholus is a small domical mountain or hill. On Mars they are named after the nearest named albedo feature on the Schiaparelli or Antoniadi maps.