(CNN) -- A runaway balloon has touched down in Colorado after a 6-year-old boy untied it from his family home in Fort Collins. The dome-shaped balloon is 20 feet long and 5 feet high, Larimer County Sheriff's Office says. The boy was not inside the helium aircraft when it landed, CNN affiliate KMGH reported. A sibling saw the boy get into the craft Thursday morning. Officials were concerned that the boy may have fallen out of it, an undersheriff said. Margie Martinez of the Weld County Sheriff's Office said a sibling saw Falcon Heene climb into the basket before the balloon took off from his parents' Fort Collins, Colorado home. Since the door on the balloon was unlocked, it's possible the boy had fallen out, Martinez said. The balloon appeared to be a saucer-shaped, Mylar-coated helium balloon, similar to a party balloon. The craft was drifting eastward, authorities said. Watch the balloon float thousands of feet over Colorado » The helium balloon was tethered to the boy's family home, the Larimer County Sheriff's Department said. The boy got into the craft Thursday morning and undid the rope anchoring it. The department said the dome-shaped balloon is 20 feet long and 5 feet high. "The structure at the bottom of the balloon that the boy is in is made of extremely thin plywood and won't withstand any kind of a crash at all," said Erik Nilsson, Larimer County emergency manager, according to CNN affiliate KMGH. A dispatcher received a call Thursday morning, and emergency services personnel were contacted, Larimer County Sheriff's Department spokeswoman Kathy Davis said. http://www.cnn.com/2009/US/10/15/colorado.boy.balloon/index.html What a terrible story. I hope hes OK.