unity – A terrain with underground tunnels

The Terrain tool in Unity is great for constructing 3-D landscapes that are open from above. I am working on a game that happens entirely inside tunnels (specifically, blood vessels in a human body. But it can also be tunnels in an ant nest, or sewers below a city). There is nothing “overground” – all movement is inside the tunnels. I am looking for a tool, similar to Terrain, for easily constructing this complex system of tunnels.

One solution I thought of is to construct a terrain with “half tunnels” (i.e., open from above), then duplicate it, rotate 180 degrees, and “glue” on top of the first copy. But it can only handle one layer of tunnels, and in addition, for some reason I could not rotate the terrain.

Is there a way to construct such underground tunnels with the Terrain tool, or with a different tool in Unity?