Habitat(s)

If we knew there was at least one substantial cavern on Mars, I would think the right approach would be to send people & equipment, with temporary shelters, to attempt to airlock the cavern for use as a permanent habitat.

Define substantial please. volume/area/meters/inches/whatever

johno

I'm thinking 25,000 sqft of living/social/workspace, minimum.

You should read this:
http://www.norwebster.com/mars/lavatube.html
You should rethink whatever plans you're working on. 2,500 sqm will be barely enough for power generation, food growing, waste recycling, living and working space for 1 person. Finding a tiny cavern like you mention will be much harder than finding large lavatubes. They have already been located and photographed from space. We have even identified a few entrance points.

johno

Maybe better to send a minimal habitat to orbit around Mars, near one of its moons. Bootstrap it up to full habitability using materials from the moons, while we're mining them for volatiles for fuels and life-support.

Consider that a habitat which spins for Mars G en-route, is taken down and stowed for aerobraking into Mars orbit, then re-built with longer and more massive truss/cable structure for full Earth G, with full regolith radiation shielding for complete shielding from cosmic rays.
Either of these options would be too massive for the space transit, but are desired for long-term habitation. Full Earth gravity is almost impossible on Mars, and full radiation shielding requires you to go underground, which also means bringing along massive and complicated power systems. In free space, all you need is a mirror pointed at the Sun (and metal foil mirror is simple for an early space facility.)

Truss and cable and rock-dust shielding (perhaps an erzatz "concrete") is also fairly simple for an early boot-strapped space facility. Easily automated, low crew intervention needs.

Now crews no longer need to rotate back to Earth to rehabilitate from long times spent n low-G and high rads. We also have a source of water and ices in orbit, and getting things back from Mars' moons is easier than bringing it up from Earth, more economical than our Moon.
And we've got a robust base at/around Mars.
Tele-robots can be sent down to explore and assist crews, piloted from above, and supplies could be dropped down to anywhere on the planet (maybe via rotavator?)

Maybe far better for the first missions to Mars maybe even not plan on going down to the planet. Rather, set up the "seed" of the space facility, to begin production of fuels & water to ship back to Earth orbit with the first crew rotation (Idea stolen from Dr.Brian O'Leary in his book "Mars 1999")
Water will be worth its weight in gold in Earth orbit for a long time to come, and is perhaps more easily gotten to us from Mars' moons than anywhere (see links). Tele-robots scout the planet and search for a prime spot for crews to be needed t explore for possible signs of life or easily gotten water.

The Deimos Water Company
http://www.spacefuture.com/archive/the_ ... pany.shtml

Near Earth Object fuel
http://neofuel.com/index_neofuel.html

Bootstrapping Space Communities with Micro Rovers and High Tensile Boot Laces
(Tethers)
http://www.marshome.org/files2/BruceBootstrap.pdf

Projects to Employ Resources
of the Moon and Asteroids Near Earth
in the Near Term
http://permanent.com/

I still like the solution Isaac Asimov proposed in "The Martian Way." Use moonlets from Saturn (or possibly Jupiter?). We already know there are pieces of ice anywhere from dust-sized to moon-sized (multiple tens of kilometer diameters). Instead of using manned craft to retreive the ice, send a robot. Very cheap, a long-duration mission is no problem, and the ice itself becomes the reaction mass to bring it in to Mars, the Moon, or Earth orbit. If we started in a couple of years, we could have a 10-kilometer chunk of ice dropped on the Moon right next to where we want to set up our base, by the time we are ready to send people there. Simple, cheap, elegant.