[freebeard]

'Terraforming' Mars is just trading one gravity well for another.

More interesting to me is Phobos.

en.wikipedia.org: Shklovsky's "Hollow Phobos" hypothesis

Shklovsky's "Hollow Phobos" hypothesis
In the late 1950s and 1960s, the unusual orbital characteristics of Phobos led to speculations that it might be hollow.[40] Around 1958, Russian astrophysicist Iosif Samuilovich Shklovsky, studying the secular acceleration of Phobos' orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin.[41] Shklovsky based his analysis on estimates of the upper Martian atmosphere's density, and deduced that for the weak braking effect to be able to account for the secular acceleration, Phobos had to be very light—one calculation yielded a hollow iron sphere 16 kilometers (9.9 mi) across but less than 6 centimetres (2.4 in) thick
...
Subsequently, the systematic data errors that Singer predicted were found to exist, the claim was called into doubt,[44] and accurate measurements of the orbit available by 1969 showed that the discrepancy did not exist.[45] Singer's critique was justified when earlier studies were discovered to have used an overestimated value of 5 centimetres (2.0 in) per year for the rate of altitude loss, which was later revised to 1.8 centimetres (0.71 in) per year.[46] The secular acceleration is now attributed to tidal effects, which create drag on the moon and therefore cause it to spiral inward.[47]

The density of Phobos has now been directly measured by spacecraft to be 1.887 g/cm3 (0.0682 lb/cu in).[48] Current observations are consistent with Phobos being a rubble pile.[48] In addition, images obtained by the Viking probes in the 1970s clearly showed a natural object, not an artificial one. Nevertheless, mapping by the Mars Express probe and subsequent volume calculations do suggest the presence of voids and indicate that it is not a solid chunk of rock but a porous body.[49] The porosity of Phobos was calculated to be 30% ± 5%, or a quarter to a third being empty.

Much more achievable than colonizing the planet or O'Neill Cylinders.