Located outside the town of Coolidge, this national monument preserves one of the most unusual Indian ruins in the state. In Spanish, casa grande means "big house," and that's exactly what you'll find here. In this instance, the big house is the ruin of an earth-walled structure built 650 years ago by the Hohokam people. It is speculated that the building was once some sort of astronomical observatory, but this is not known for certain. Whatever the original purpose of the building was, today it provides a glimpse of a style of ancient architecture rarely seen. Instead of using adobe bricks or stones, the people who built this structure used layers of hard-packed soil, which have survived the ravages of the weather and still stand in silent testament to the Hohokam's long-ago architectural endeavors. The Hohokam began farming the valleys of the Gila and Salt rivers about 1,500 years ago, and eventually built an extensive network of irrigation canals for watering their fields. By the middle of the 15th century, the Hohokam had abandoned both their canals and their villages and disappeared without a trace.