The Five Types of Tunnels

When we talk about drains there are five basic types. Let’s run through them and see what each one is all about.

Storm Drains aka Storm Sewers

A Storm Drain

These pipes transport surface water from streets, parking lots, fields, and yards to nearby rivers or in some areas for treatment. The end of a storm drain is called the “outfall” and can usually be found connecting and flowing into to a river. Storm drains are typically their largest size at the outfall and get smaller the further in you travel. For most tunnel explorers, these are the type you will want to focus on. But before you do, please read our article “So you want to explore a storm drain”.


A Sewer

These are pipes whose primary function is the transportation of raw sewage, meaning direct unprocessed liquid and solid matter, consisting mostly of water. These can be low or high-pressure. All sewers of this type will be filled with a collection of very dangerous gases. Do not enter under any circumstances, you will likely lose your life.

In some parts of Milwaukee, sewer pipes will connect into storm drains for controlled overflow in emergency situations. These areas are referred to as a CSO meaning “Combined Sewer Overflow”. Because these discharges of raw sewage are remotely controlled with a mouse click and can happen unexpectedly, experienced drain explorers avoid these. The MMSD publishes maps showing CSO discharge areas.

Steam Tunnels

A Steam Tunnel

A classic kind of tunnel that many explorers have encountered. For our purposes here in Wisconsin, there are two types of steam tunnels. Those located on the grounds of an existing or former university/institution and those owned and maintained by WE Energies. Old steam tunnels can be fun, exciting, and can be found all over the state if you do your homework. The WE Energies tunnels are fully active and workers use them three shifts a day. You can expect the tunnel to be between 125 and 150 degrees depending on your location. Long pants and sleeves are required and nylon or other meltable clothing is not recommended. Remember, the ladder will also be over 125 degrees so wear gloves. Entering these tunnels is also illegal and WE Energies will prosecute.

Inter-building tunnels

An Inter-Building Tunnel

These are some of the toughest to find but oh so cool. Originally used for convenience especially during winter, and a smaller number that served as prohibition-era transport tunnels. You will find them under churches, neighborhoods near convents, universities, and in many parts of the greater downtown area. This category also covers beer caves which are small caves, often hand dug used to store beer prior to modern cooling.

Deep Tunnel

The Deep Tunnel

Like many cities worldwide, Milwaukee has a “deep tunnel” system. Consisting of nearly 30 miles of tunnel the system can store around 525 million gallons of combined sewage and surface water in an emergency. Before the tunnel existed, discharges of raw sewage into Lake Michigan were frequent and substantial. Deep Tunnel operations have reduced these discharge events to a fraction. These tunnels are located 300’ underground and cannot be reached without the use of a crane and a multi-day venting of the area beforehand.

Note from the editor: Some of the information contained in this article is applicable only to Milwaukee. Before exploring, always be sure to check local laws, regulations, and practices to ensure the safety and legality of your travels.