Underground tunnel?

The discoloration at the bottom indicates that it occasionally floods with something, so it is likely a culvert or drain of some sort.  It could have been "dirty water" domestic outflow or simply clean water that wet the bricks and then grew mold.

Did you call the 800# before digging......they would have told you.

Someone could run a camera through it to find out where it goes and map it.

Could be for water supply or waste drainage.

Is the black some kind of tar?

Or just water stains?

Perhaps call a local Museum to see if they know anything?

Archaeologists might be interested.

City hall knows or your water utility knows.  They know about the land underground so they know every underground spring or swamp or one time ditch.   I had a water issue and went to water utility and they had a map of my area .   they knew of creeks or springs before they closed them all up with top soil.  As it is red clay brick it is not Roman but there was a liquid in there and being that it was covered over I am thinking sewage water like a "weeping tile" as the water would eventually seep into the ground  to be filtered out. In the early days they did not have running water so dirty water was either thrown out to evaporate in the sun as the water was drawn from a well by a bucket.  Bathing was once a month.  No indoor plumbing so chances are good there were several outhouses on the property over its lifetime.  As one was getting half full then dirt was dumped on top of that brown stuff and a new hole was dug several feet away. hole would be 6 or 8 feet deep so it take awhile to fill with solids half way up.     Water utility would not know of the outhouses because that is not a water source but a small creek or diverting a small creek would be something they would have done and know about. Because it is black, tells me it was not clean water.  Scrape off some of the black stuff and take it to a lab and have it analyzed.   Pooh is the most likely result.

its a roman aqueduct 

from when they civilized britain

It's a culvert as a previous person has said.  Have a nosy and see what was in the area before your house was built:

https://maps.nls.uk/os/6inch-england-and-wales/  

It will be draining toward some watercourse.

Victorian drain ..all early Victorian drains were built of brick ..before that there were none

You can't spell. 

Wow! Took some craftsmen or craftswomen to build that. Please update with size and floor pitch. Is it level at the floor?

Looks like a drainage culvert. Maybe you should see what was on the site before your house was built.