Cool image time! The image on the right, reduced to display here, was taken by Mars Reconnaissance Orbiter and shows a very intriguing drainage pattern with several different meandering washes apparently flowing into what clearly looks like a sinkhole. As noted at the MRO website,
The drainages in this image are part of Hebrus Valles, an outflow channel system likely formed by catastrophic floods.
Hebrus Valles is located in the plains of the Northern lowlands, just west of the Elysium volcanic region. Individual channels range from several hundred meters to several kilometers wide and form multi-threaded (anastamosing) patterns. Separating the channels are streamlined forms, whose tails point downstream and indicate that channel flow is to the north. The channels seemingly terminate in an elongated pit that is approximately 1875 meters long and 1125 meters wide. Using the shadow that the wall has cast on the floor of the pit, we can estimate that the pit is nearly 500 meters deep.
To better see the shadowed floor of the sinkhole, I have cropped the full resolution image to focus on the pit itself, and increased the brightness significantly as shown on the right.
There does not appear to be any openings in the sinkhole floor into which water could drain, a lack that can be explained several ways. First, it is not unusual on Earth for sinkholes to have floors like this, without any obvious drainage opening, and for the water to still sink easily into the sand and rocks. Second, even if the drainage here is completely blocked, this could merely mean that it has been a very long time since water flowed here. When similar drainage systems dry up here on Earth, they fill with sediment and debris over time, so that all access to the cave below gets blocked. Many of the cave dig projects that I have been involved in over the last few decades were aimed precisely at digging out this debris to get access to the virgin cave below.
Either way, this Martian drainage indicates the presence of underground hydrology. Or to put it another way, water that future colonists can drill down into and access.
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