They were probably built by the same lot that did Stonehenge. Elvis lives there too. And if you multiply the number of pyramids by pi and divide by the circumference of the Sphinx you'll end up with a number that's pretty close to the date of the foundation of the Knights Templar...

Sorry LH, just getting in before Wolfgang does, if I can type fast enough.

Yeah, there are shapes in photographs of Mars that look like ruins - and apparently a face and a pyramid. then again, the human visual system is pretty damn hot at object and pattern recognition (even where there's none) - and at projection of ideas. I'd guess that shadows and angle and range and scale all have a part to play too.

A Google search on "ruins" and "mars" brings up at least 25700 pages...
I liked this page, and here's the official one - but don't expect much - it's a conspiracy, remember!