A haunted house or ghosthouse is a house where supernatural events are believed to take place. Many haunted houses are believed to contain ghosts. They can also contain the spirits of dead people. The rumour that a house is haunted often starts after something violent or tragic happens in the house, such as a murder or suicide. Haunted houses are common in gothic fiction, and are generally seen as old castles or creaky, old, abandoned houses. A haunted house can be a type of attraction in amusements parks too. One scientific explanation of haunted houses is that poisonous gasses such as carbon monoxide from defective gas-burning appliances are what causes the hallucinations that people experience. Another possible cause is infrasound.