It's what is known as an Underground Cable Vault (UCV). I work with them every day.
Under the manhole cover is a square vault, usually about 10-12 feet on a side. Through it runs fiber optic cabling. Inside there is probably what is
known as a "service loop" tied to iron racking on the walls of the vault. The service loop provides slack in the cable should it ever be cut or need
to be spliced.
If you were to open the manhole there's probably better than a 70% chance all you would see would be water. Most of them are full of water (the
cables are designed to be in submerged environments indefinitely).
AT&T is by no means dead. They are very much alive. They don't sell residential telephone service anymore, but they remain a mammoth international
carrier for voice and data services around the globe. In fact, AT&T is so mammoth that most major telephone companies lease bandwidth on their
cables. Many telcos would not be able to even function if it were not for AT&T's cabling infrastructure across the US and many parts of North
America. AT&T is the World's largest telephone provider, by orders of magnitude (even to this day).
If you were to dig down along side of this manhole you would encounter a metalic plastic tape with some serious warnings on it, called "Scare Tape",
at about 3 feet below the surface. Below this you would likely find a series of 4" conduits buried in red colored concrete. And if you were crazy
enough to ever attempt to cut one of these cables there would be helicopters on top of you in a matter of minutes. The fiber optic cables run in
bundles of 144 and 288 strands of "single mode" fiber optic cabling (inside of a single jacket). One of the fibers inside these cables (about the
size of a human hair) is dedicated to an "Optical Time Domain Reflectometry" function which will zero in on the location of a cable cut (down
literally to the inch) from 300 miles away.
And yes, cable damage to one of these cables really can cost upwards of $1 million dollars a minute! It's not a myth. And, repairing one is not
easy. Some repairs can take days. I know (personally) of a large earthwork company who went bankrupt overnight after cutting a transcontinental
ETA - Current technology called "Distributed Acoustic Sensing" monitors cable disruption. If you were to just put a wrench on one of the bolts on
that manhole cover people in AT&T's operations center would know. If you were to pound on the lid with a sledge hammer or large rock there would be a
helo in the air in minutes to come check it out with an overflight. You might ask yourself how then can cables which run under busy roadways not set
off alarms all the time? And the answer is, because the technology can be 'attenuated' to ignore normal noises and vibrations and only alarm on
Note: Your manhole find is actually surprisingly well marked. Most are not.
Fun Fact: Do you know where you can almost always find a SPRINT fiber optic trunk line? Answer: along side a main railroad line. The "SPR" of the
word 'SPRINT' stands for "Southern Pacific Railroad".
edit on 7/14/2019 by Flyingclaydisk because: (no reason given)