There was a lateral tunnel.
It seems like it's a cave.
Its height is about 2 meters, its width 3.
A similarly sized passage ran right beside it.
Water is flowing through the ground, reinforced by masonary stones; it's a tricklying stream but it doesn't feel like it's sewage.
The passage we just came through simply ends in this cave.
We are in a slightly higher position, there is about a one meter drop to the cave floor.