A private not-for-profit organisation dedicated to encouraging the exploration and settlement of Mars. Founded by Robert Zubrin and others in mid-1998 and attracting the support of notable science fiction writers and filmmakers (including Kim Stanley Robinson and James Cameron), the organisation is dedicated to convincing the public and governments of the benefits of Mars exploration, as well as exploring the possibilities of private Mars missions. The Society has thousands of members across many countries.
The Society's most notable private research project to date involves a "simulated Mars habitat" on Devon Island, an island in the Arctic with geography and weather conditions resembling some aspects of Mars. At this research station they have tested communication, command, scheduling, and scientific investigation techniques and approaches under simulated Mars conditions (for instance, each exit from the main habitat requires the wearing of a pseudo-spacesuit and a 20-minute prebreathing period, as would be necessary on Mars). At a cost of over 1 million US dollars, the society has demonstrated considerable fundraising and technical capabilities (though, of course, a small fraction of the billions of dollars required for an actual manned Mars mission).
The Mars Society has announced their next major project involves investigating the biological effects of extended exposure to centifugally-generated Martian "gravity" on mice.
The society's web site can be found at http://www.marssociety.org