In 1977, an Illinois man built an underground house with rocks he collected on trips to Colorado to fight rising energy costs; nearly 50 years later, the 2,300-sq-ft 'caveman' home is listed for just $59,000

• Pic credit: Midwest Real Estate Data LLCIn the 1970s, when energy costs were climbing, an Illinois man went underground.
• The earth-sheltered house he built at 508 S Jefferson Street in Armington, Illinois, hit the market on Thursday for just $59,000.
• The home has already secured a place on this week's most popular homes list, according to Realtor.com.