Tunneled domino solutions by Michael Reid.
+---|---+-------+-------+-------+
|   |   |               |       |
|   |   |               |       |
|   \-----------------------U   |
|       |               |       |
| 4     | 1             | 6     |
+-------+-------+-------+-------+
|       |       |               |
|       |       |               |
|   U-----------------------U   |
|       |       |               |
| 7     | 5     | 8             |
+-------+-------+-------+-------+

+-------+-------+-------+-------+
|       |               |       |
|       |               |       |
|       |           /-------D   |
|       |           |   |       |
| 4     | 2         |   | 6     |
+-------+-------+---|---+-------+
|       |       |   |           |
|       |       |   |           |
----D   |       |   \-------D   |
|       |       |               |
| 7     | 5     | 8             |
+-------+-------+-------+-------+

 +-------+-------+-------+
|               |       |
|               |       |
--------------------\   |
|               |   |   |
| 1             |   |   |
+-------+-------+   |   +
|               |   |   |
|               |   |   |
|   U---------------/   |
|               |       |
| 3             | 6     |
+-------+-------+-------+
|       |       |       |
|       |       |       |
|   U   |       |       |
|   |   |       |       |
| 8 |   | 5     | 4     |
+---|---+-------+-------+

+-------+-------+-------+
|       |       |       |
|       |       |       |
|   /-------\   |       |
|   |   |   |   |       |
|   |   |   |   |       |
+   |   +   |   +       +
|   |   |   |   |       |
|   |   |   |   |       |
|   D   |   \-------\   |
|       |       |   |   |
| 8'    | 7     | 2 |   |
+-------+-------+---|---+
|       |       |   |   |
|       |       |   |   |
|   D---------------/   |
|       |       |       |
| 8     | 5     | 4     |
+-------+-------+-------+