Urban exploring is anything to do with exploring your city and manmade structures, often shortened to UE or urbex. It could be abandoned building, mining shafts, or drainage tunnels. This is also referred to as 'draining' and it can be quite dangerous. Tunnels can collapse, stormwater drain can flash flood and have bad air, and abandoned building may expose you to harmful chemicals and asbestos dust. Other possible hazards are things like discarded syringes, rusty and sharp objects, slipping, falling, and unstable nature of old builds. The activity presents various risks, including both physical danger and, if done illegally and/or without permission, the possibility of arrest and punishment. See also the Wikipedia entry on urban exploration.
The catacombs found in Paris are a prime example of this. However, local examples include the mine shafts at Paluma, the Hervey Range Train Tunnels, WWII bunkers, train yards, and storm water drains. There are also parkour groups that explore rooftops and construction sites, also known as 'rooftopping' or 'skywalking'.
Photographers are also looking for unique and grungy locations. In 2018, some also use quadcopter / drones to fly through and close to building. Location-based games such as Ingress and Pokémon Go also incorporate urban exploration to some extent. The geocaches has also hidden some items around the stormwater drains of Townsville.
For stormwater drains, the entrance will start at a waterway (canal, creek, river, lake, beach etc). Sometimes the drain will be made out of a variety of different materials, the most common is concrete but also could be other materials such as redbrick, bluestone, plastic, corrugated steel, sandstone, and even carved through natural rock. Occasionally, you will come across animals and insects such as cats, bats, and cockroaches. After a while the tunnel will more then likely do one of two things; turn back into a creek, or shrink down into an awkward height. The latter are called 'shrinkers'.