What happened to Mars' oceans?

The Mars ocean hypothesis

Mars, commonly referred to as the 'Red Planet', is known for it's harsh environment and frigid temperatures, hostile to life though the possibility of microbes on Martial soil still exists. However, less generally known, it is widely believed that at one point of it's existence, approximately 3.8 - 4.1 billion years ago an ocean covered nearly 33% of it's surface.

Mars as we see it today and what it could have possibly looked like based on geological data

The primeval ocean, named the Paleo-Ocean and Oceanus Borealis, would have covered the entire Vastitas Borealis basin, the largest lowland region of Mars. This basin lies roughly 4-5 km below the average surface elevation of the planet.

As Marcello Truzzi once stated, "extraordinary claims require extraordinary evidence". Although depicting the Mars Ocean hypothesis as an 'extraordinary claim' is perhaps a bit too much, evidence supporting the aforementioned is plentiful, based off of geological data.

The evidence

The Viking Program which took place in the late 1970s consisted of an orbiter and a lander, showed evidence of ancient shorelines thousands of kilometres in length. Furthermore, ravines formed by the flow of water merge into larger channels imply soil erosion by a liquid substance. These ravines and channels resemble ancient riverbeds on earth.

Moreover, enormous channels each tens of kilometres wide and hundreds of metres below the surface appear to transmit water from aquifers (permeable rock that can carry/displace water) from the southern to the northern hemisphere. The latter is also at a lower elevation than the rest of the planet, suggesting the existence of an ancient sea.

95.32% of Mars' atmosphere consist of CO2 (carbon dioxide). Therefore it is expected that carbonate minerals might be found in Martial soil, although evidence of such has not been found just yet. However, that logic is partially flawed as it assumes the primordial ocean was non-acidic. Evidence of this claim, however has been found. Positive correlation of elements such as chlorine and sulphur on Mars' surface were found at landing rovers' sites, indicating acidic reservoirs.

What happened to it?

To answer the title of this post, there are several theories to explain the disappearance of the oceans. As Mars is quite a frigid environment, it is suggested that as Mars cooled down the oceans froze and are hidden beneath a layer of rock and debris in the northern hemisphere basin Vastitas Borealis, since that is where the ocean is though to have formed. Furthermore, additional theories indicate that water could have been lost to the atmosphere through sublimation (when a liquid turns into a gas) and subsequently to space.