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Unfortunately Drupal only allows you to add primary and secondary menu’s by default. However there are times you need deeper levels, for example a third (tertiary) level submenu.
After some searching and looking at the core code I came up with a pretty easy solution.

First add the following functions to your template.php file.

function phptemplate_preprocess_page(&$vars) {
    $vars['tertiary_links'] = menu_tertiary_links();
}

/**
 * Returns an array containing the tertiary links based on the primary menu.
 * Tertiary links can be either a third level of the Primary links
 * menu or generated from a second level of the explicitly defined secondary_menu
 */
function menu_tertiary_links() {
    if (variable_get('menu_secondary_links_source', 'secondary-links') == variable_get('menu_primary_links_source', 'primary-links')) {
    return menu_navigation_links(variable_get('menu_primary_links_source', 'primary-links'), 2);
  }
  else {
    return menu_navigation_links(variable_get('menu_secondary_links_source', 'secondary-links'), 3);
  }
}

Once you’ve done that, you can add this snippet in your template.

<?php if (isset($tertiary_links)): ?>
    <?php print theme('links', $tertiary_links, array('class' => 'links tertiary-links')); ?>
<?php endif; ?>

And voila! you’re done.