Reports of hauntings include seeing a figure, odd smells, sudden changes in temperature and a strong sense of presence. Each of these is something a person truly experienced, and each is a report. Many people hold the idea. A 2001 Gallup poll found about 38 percent of Americans believed ghosts exist, and in a June 2005 Gallup phone poll of 1,002 adults, 37 percent said they believe houses can be haunted. A poll counts what people believe. Evidence asks what else could produce the same experience, and whether anyone can check it for themselves, and the same question can be asked of faces, shadows, stage tricks, houses and sound.

The Society for Psychical Research was founded in London in 1882, to study such reports with care. It grew out of a conference held in London in January 1882, and its first president was Henry Sidgwick. A learned society is a group of scholars who agree to examine a subject by shared rules. Its aim was to study such reports with care, which means asking what happened and what could explain it.

Seeing faces in things that have no faces has a name: pareidolia. People see faces in clouds, rocks and tree bark. Babies are attracted to faces from birth, even though their eyesight is poor. The visual system groups small parts, like a mouth and eyes, into a bigger picture, which is how it can build a face out of clouds or bark, and the brain area that notices faces gets busy when we only think we see a face. One research team says the brain evolved to help us deal with other people, and that shapes how we see the world, so we see faces in objects.

A hill on Mars looked like a face in one old photo because of shadows. The hill is 1.5 kilometers (one mile) across, and the Viking photo was taken with the sun about 20 degrees above the horizon. The speckled look of the first Mars picture came from bit errors in the data, made more obvious when the photo was enlarged. NASA says the hill looks like a face only in low-resolution pictures with very specific lighting. Sharper photos showed it is just a hill, and the hill has dozens of hills like it nearby. Many rocks on Mars were sculpted by wind erosion, which is one reason odd shapes turn up there.

Henry Dircks first made the glass effect by accident, around 1858. John Henry Pepper, who ran the Royal Polytechnic in London, turned the idea into a stage effect in 1862. People made a see-through ghost with a sheet of glass and a hidden actor in bright light. Pepper's ghost used a bright light on a hidden actor and a big angled pane of glass, so the audience saw the actor's reflection on the stage. It became the talk of Victorian London, and the Prince of Wales came to see it in 1863.

Houses make creaks and pops. Wood, metal and glass get a tiny bit bigger when they warm up and a tiny bit smaller when they cool down. Building materials in a house, such as wood, steel, masonry and glass, expand in the heat of the day and contract again at night when it is cooler. Cold pipes can make noise when warm or hot water runs through them. Winter air dries wood out, so the boards shrink. That is why some floors make more noise in cold weather. Drafts around windows and doors get noticed, but in most homes the biggest air leaks are hidden in the attic and basement.

People generally hear sounds between 20 Hz and 20,000 Hz, and infrasound is below that range. It is sound with a pitch below 20 hertz, lower than the sounds people usually hear. Infrasound is wrongly thought of as completely inaudible, because people can perceive it when it is strong enough. It comes from nature too: volcanic eruptions, earthquakes, falling meteorites, tornadoes and lightning make it. Vic Tandy was one of several scientists working in a medical equipment laboratory when strange feelings began, and he traced the eerie feelings to a faulty extractor fan that made very low sound.

Scientists in England and Scotland asked visitors to walk around places with a haunted reputation and mark where they felt anything unusual. That turns a feeling into a record, and a record can be compared from one room to the next. A creak has a cause in the wood, a face has a cause in the brain, and a figure on a stage had a cause in the glass. A strange feeling in a room is worth taking seriously, and it is also worth asking careful questions, one room and one explanation at a time, so that a feeling can be matched with a cause that other people can examine as well.

Words to know

• pareidolia: seeing faces
• infrasound: very low sound
• society: group of scholars
• evidence: checkable proof