Collapsed Tunnel
================

+-- Ground level -----+
|                     |
|     <C>             |
|                     |
|                     |
|                     |
|                     |
|         ^^          |
|      *--M           |
|     # #             |
|    /   \            |
|   M     *--*--*     |
|                \    |
|                 \   |
|      *--*        *  |
|     <    \      /   |
|           \    /    |
|            *--*     |
|                     |
+---------------------+