Tholus (37.3°S, 165.1°E) is a very unusual volcanic feature on
Venus; in fact, only one other feature, located in Ovda Regio, has similar
morphologic features. The bright, ridged flows stand about 600 meters
above the surrounding plains, and the inner tier sits over 1000 meters
high. The thickness of these flows and their several-hundred-meter ridge
spacing suggest they were unusually viscous at the time of emplacement.
No suitable explanation has yet been presented for why this feature formed
in otherwise featureless plains.