The BBC is reporting on peer-reviewed articles in Nature, which document a large frozen sea and geologically-recent volcanic activity.
The interesting part about this is that according to physicists, the reason Mars lacks a more substantial atmosphere is that its core has cooled and solidified. Such a core cannot generate the magnetic field needed to shield the atmosphere from solar wind. Unprotected, the atmosphere literally gets blown away.
But apparently “Venus has no magnetic field, perhaps because of its slow rotation.” Like earth, Venus still has a molten core. Volcanism on a planet like Mars, presumed to have a cold solid core, is a little harder to explain.
If Mars has a warmer core than anyone expects, then the absence of a magnetic field is also apparently unexplained. And since the lack of a magnetic field explains the relative lack of atmosphere, then we also don’t know why or if Mars can’t be terraformed.