Saguaro cactus

Cactus fascinates me. All my life I've lived around vegetation that generally gets more than .01 inches of rain per year, and cactus seems to thrive in the driest climates on the planet. Here's the iconic saguaro cactus::


There were so many cactus in the area, they looked like motionless beacons left by an alien civilization:


Some barrel cactus, I believe::




And the prickly pear cactus::




Not sure if this organ-pipe or saguaro cactus, but I do know that city beyond is Tucson, AZ::