What i would do is this:
Use the level tool to make a canyon and line it with imported tunnel sections, then import your premodelled tunnel mouths and the drag tool to close the terrain just around the mouths of the tunnels. You can also model the entire tunnel as one unit and import it as a prop too if you want to. Dont forget you can re-import ted made mini-sections (eg, 16x16) as props which may be useful too :)
If you are talking about making holes in the terrain then unfortunately that is not possible in the editor itself except for where there is a seam (between 2 blocks). In this instance you can use the drag tool to open a hole.
Use the level tool to make a canyon and line it with imported tunnel sections, then import your premodelled tunnel mouths and the drag tool to close the terrain just around the mouths of the tunnels. You can also model the entire tunnel as one unit and import it as a prop too if you want to. Dont forget you can re-import ted made mini-sections (eg, 16x16) as props which may be useful too :)
If you are talking about making holes in the terrain then unfortunately that is not possible in the editor itself except for where there is a seam (between 2 blocks). In this instance you can use the drag tool to open a hole.