Dark Grey Stone Tunnel (3)
==========================

+-- Ground level --------+
|            ^           |
|            *           |
|           / \          |
|          /   \         |
|         *     *--*     |
|          \      / \    |
|           \    /   \   |
|            *--*     *  |
|           /         |  |
|          /          |  |
|      *--*           *  |
|      |             /   |
|      |            /    |
|      *        *--*     |
|     / \      /         |
|    /   \    /          |
|   *     *  *           |
|    \        \          |
|     \        \         |
|      *        *        |
|      |        |        |
|      |        |     ^  |
|      *--*     *--*--%  |
|                        |
+------------------------+