Slightly northwest of the Lowell Boulevard and the Trails Ave. intersection in Broomfield, CO there's a little complex comprising of 6 drains, 4 of which go to the same location. I have not explored the other two as of of yet due to poor conditions. Regarding the 4 drains leading to the same destination, there's a pair of high drains, and a pair of low ones roughly 3 feet high (despite the short stretch of only around 300 feet, it will give your thighs a good workout!) To the Right of the 2 sets of drains there's a black corrugated pipe that is roughly the same size that leads southeast and is believed to run underneath the nearby neighborhood and connect to the storm drains. Around 100 feet East of the 5 drains, there is another one, it's roughly 6"-1' bigger, it leads somewhere under the little trail between the two neighborhoods, probably more northeast.
A View of the 5 close together drains
Inside the drain (Second from the left.)
The "Party Room" as named by me and my buddy, "Abandonment_Traveler", you can see graffiti throughout this room, its about 5 feet long, 15 feet wide, and 15 feet tall and usually pretty dry.
A plastic ladder in the "Party Room" that leads to nowhere, it's handy to climb down if the floor is wet (The manhole is on the
other side of the room.)
Ah, last but not least, a small tributary drain only about 3" wide, it's always flowing a little bit. I am fairly sure the brown color is just sediment, but I didn't particularly want to touch it to find out. Here you can see another plastic ladder leading to nowhere.