In 2004, three different research groups independently announced the discovery of significant concentrations of methane in the atmosphere of Mars. But the predicted lifetime of methane in the Martian atmosphere has been calculated at just 200 days (Source : Nature, 460(7256):720-3(paywalled) ) - so it appears that the gas must be being constantly generated by as yet unknown mechanism(s) - possibly geochemical processes, volcanic or hydrothermal activity, or living organisms.

It's speculated that living organisms could generate the methane either as a waste product, or by decomposition. The current NASA Perseverance mission will attempt to sample the Martian soil in search of evidence.

Further reading: Methane on Mars The Economist^{[ paywalled ]}

Also see :Mars Oceanplugin-autotooltip__plain plugin-autotooltip_bigMars Ocean

Many of the extensively surveyed geological features of Mars suggest that the planet once had an extensive liquid water ocean.

The theoretical ex-ocean has been named as Paleo-Ocean and Oceanus Boreali, and some researchers suggest that it could have covered up to 30% of the planet's surface. The concept, however, is not yet universally accepted.