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     | +-------+-------+-------+