Urban Exploration
(often shortened as urbex or UE) is the exploration of man-made structures, usually abandoned ruins or not usually seen components of the man-made environment. Photography and historical interest/documentation are heavily featured in the hobby and, although it may sometimes involve trespass onto private property, this is not always the case and is of innocent intention. Urban exploration is also commonly referred to as infiltration, although some people consider infiltration to be more closely associated with the exploration of active or inhabited sites. It may also be referred to as draining (when exploring drains) urban spelunking, urban rock climbing, urban caving, or building hacking.

- Philosophical and psychological aspects:
Several writers on urbex have discussed the personal meaning of such acts of "infiltration" - or "invasions". Simon Cornwell, in his discussions of the Cane Hill Cult (in Croydon, South London), has emphasized the element of danger in recording the experiences - physically, emotionally and photographically. This element of danger serves to heighten the existential anxiety of exploration. Here, the task is akin to psychological recovery, and the mental hospital, so long a refuge for the discarded and the dispossessed, is now itself discarded. In this context, the work of the explorers is akin to that of psychotherapists in their exploration of psychological structures excited by historic traumas.

- Abandonments:
Ventures into abandoned structures are perhaps the most common example of urban exploration. At times, sites are entered first by locals and may sport large amounts of graffiti and other acts of vandalism. Explorers face various risks in abandoned structures including collapsing roofs and floors, broken glass, guard dogs, the presence of chemicals and other harmful substances (most notably asbestos), hostile squatters and motion detectors. Some explorers wear respirators to protect their airways and proper attire to protect their bodies.

Although targets of exploration vary from one country to another, high-profile abandonments include amusement parks, grain elevators, factories, power plants, missile silos, fallout shelters, hospitals, asylums, schools, poor houses, and sanatoriums.

In Japan, ruins are known as haikyo (廃虚?), (literally "abandoned place") but the term is synonymous with the practice of urban exploration. Haikyo are particularly common in Japan because of its rapid industrialization (e.g., Hashima Island), damage during World War II, the 1980s real estate bubble and the 2011 Tōhoku earthquake and tsunami.

Many explorers find decay of uninhabited space to be profoundly beautiful and some are also proficient freelance photographers. Abandoned locations can be, at times, heavily guarded with motion sensors and active security. Others are more easily accessible and carry less risk of discovery. Abandoned sites are also popular among historians, preservationists, architects, archaeologists, industrial archaeologists, and ghost hunters.

- Active buildings:
Another aspect of urban exploration is the practice of exploring active or in use buildings which includes gaining access to secured or "member-only" areas, mechanical rooms, roofs, elevator rooms, abandoned floors and other normally unseen parts of working buildings. The term "infiltration" is often associated with the exploration of active structures. People entering restricted areas may be committing trespass and civil prosecution may result.

- Catacombs:
Catacombs such as those found in Paris, Rome, Odessa and Naples have been investigated by urban explorers. The Mines of Paris, comprising much of the underground tunnels that are not open to public tourism like the catacombs, have been considered the "Holy Grail" by some due to their extensive nature and history. Explorers of these are known as cataphiles.

- Sewers and storm drains:
Entry into storm drains, or draining, is another common form of urban exploration. Groups devoted to the task have arisen, such as the Cave Clan in Australia. Draining has a specialized set of guidelines, the foremost of which is "When it rains, no drains!" The dangers of becoming entrapped, washed away, or killed increase dramatically during a heavy rainfall.

A small subset of explorers enter sanitary sewers. Sometimes they are the only connection to caves or other subterranean features. Sewers are among the most dangerous locations to explore owing to risk of poisoning by buildups of toxic gas (commonly methane and hydrogen sulfide). There have been numerous fatalities around the world[citation needed] where people are overcome by toxic gas from sewers.

- Transit tunnels:
Exploring active and abandoned subway and underground railway tunnels, bores and stations is often considered to be trespassing and can result in civil prosecution. As a result, this type of exploration is rarely publicized. One important exception to this is the abandoned subway of Rochester, NY, the only American city to have an abandoned, formerly used, subway system. (The Cincinnati subway is also abandoned, but was never completed.)

- Utility tunnels:
Universities and other large institutions, such as hospitals, often distribute steam for heating buildings and autoclaves from a central heating plant. These high pressure steam pipes are generally run through utility tunnels, which are often accessible solely for the purposes of maintenance. Many of these steam tunnels, such as those on college campuses, often also have a tradition of exploration by its students. This was once called vadding at MIT, though students there now refer to it as roof and tunnel hacking.

Steam tunnels, in general, have been secured heavily in recent years, due to their use for carrying network backbones and perceived risk of their use in terrorist activities, safety and liability.

Some steam tunnels have dirt floors, no efficient lighting and have temperatures upwards of 45 °C (113 °F). Others have concrete floors, bright light, and feature a cool low-grade temperature. Most steam tunnels have large intake fans to bring in fresh air and push the hot air out the back. Most active steam tunnels do not contain airborne asbestos but proper breathing protection may be required for other hazards. It is wise to take proper care inside active utility tunnels, since pipes can spew boiling hot water from leaky valves, puddles pool at your feet, and forceful steam may leak inward resulting in burns and slips.