In the southern hemisphere, particularly around the Hellas impact basin, are several flat-lying volcanic structures called highland paterae These volcanoes are some of the oldest identifiable volcanic edifices on Mars. They are characterized by having extremely low profiles with highly eroded ridges and channels that radiate outward from a degraded, central caldera complex. They include Hadriaca Patera, Amphitrites Patera, Tyrrhena Patera, Peneus Patera, and Pityusa Patera. Geomorphologic evidence suggests that the highland patera were produced through a combination of lava flows and pyroclastics from the interaction of magma with water. Some researchers speculate that the location of the highland paterae around Hellas is due to deep-seated fractures caused by the impact that provided conduits for magma to rise to the surface. Although they are not very high, some paterae cover large areas—Amphritrites Patera, for example, covers a larger area than Olympus Mons.