Hecates Tholus is a Martian volcano, notable for results from the European Space Agency's Mars Express mission which indicate a major eruption took place 350 million years ago. The eruption created a caldera 10 km in diameter. It has been suggested that glacial deposits later partly filled the caldera and an adjacent depression. Crater counts indicate this happened as recently as 5 to 20 million years ago. (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.