Nearly 50 miles or so southeast of Phoenix, Arizona stands Casa Grande Ruins National Monument, built by the mysterious Hohokam civilization in the early 1300s.

Archaeologists suggest that the "Great House" was an observatory of sorts, noting that the small round window on the west wall aligns perfectly with the setting sun on the annual summer solstice (June 21). Other openings line up with the sun and moon at significant dates throughout the year.

There are no stones around to build things here. So the Hohokam used a sticky mud created by mixing water with caliche ("pronounced "cuh-LEE-chee"), an underground layer of soil whose particles are cemented together by lime. A large canopy (above) now protects the structure from the elements.

Casa Grande Ruins is in Coolidge on State Highway 87, about 12 miles east of I-10 (Exit 185). Admission is $3 per person for anyone age 17 and older. It's open from 8 a.m. to 5 p.m. daily, except Christmas Day. For more information phone 1-520/ 723-3172.