Definitions:
Infall = Where the water falls into a drain
Outfall = Where the water leaves the drain
God Drain = Drain's of hazards or high difficulty
Storm Drain = Storm sewers capture rainwater or snowmelt
Sanitary Drain = Transport wastewater released from a drain, toilet, sink etc
Combined Sewers System = CSO's act as a relief valve when there is too much water in the system
Always check the weather before going so that you aren't caught in rain and die from the overflow because that has happened before to people.
Make sure you aren't picking a God drain (the drains with the ☣️ symbol), as these drains usually have some sort of hazard, or are for more advanced drainers.
When getting inside a drain, it will change depending on the drain itself. Some are just grates you have to lift, and others are pull grates. Some drains' outfalls are closed off, so you might have to go through the manholes (these are usually really hard to find). I usually tell how to get inside for each location, but if not, just try to find the outfall or the manhole.
The first and most common way is getting two people and using your middle and index fingers on both hands to pull up on the manhole.
The second way is getting a crowbar and wedging it in between and just pulling up, hoping the crowbar doesn't bend.
The third and best way is to try to find an L-shaped tool that you can hook onto the underside of the manhole and just pull up and shift it horizontally. This method usually only takes a second or two.
When inside, it's important to note that you should be wearing a sweater or something to keep from the cold as it gets really cold down in the drains. Also, make sure to wear boots for traction if the part of the drain that you're in starts to get slippery. If you ever smell anything that smells like rotten eggs, get out of there immediately, or if the air gets dense and heavy and you start coughing. This is the reason why I separate God drains from normal drains. CSO's (Combined Sewer Overflows) are combined sewers that carry both sewage and stormwater, and they cause natural gases to occur. These gases can be dangerous!
To learn more about CSOs, watch this video: Learn More
There are generally three types of drains: storm sewers (aka storm drains), combined sewers, and sanitary sewers. CSO's are usually dumped into storm sewers or occupy their own outfall; they connect to combined sewers and act as a relief valve when there is too much water in the system (e.g., a rainstorm). Each of these types comes with their own hazards and treasures.
Storm drains are typically the safest, but be careful of falling or slipping in the murky water. Also, air quality can sometimes be an issue in older sewers carrying waste, where gases like hydrogen sulfide can occur. This gas smells like rotten eggs, and it can be dangerous if you inhale too much.