Hierarchical list of taxonomy terms

gistfile1.php

function return_terms_index() {
  $taxonomies = array( 
    'taxonomy_name',
  );

  $args = array(
    'orderby'           => 'name', 
    'order'             => 'ASC',
    'hide_empty'        => false, 
    'fields'            => 'all',
    'parent'            => 0,
    'hierarchical'      => true,
    'child_of'          => 0,
    'pad_counts'        => false,
    'cache_domain'      => 'core'    
  );

  $terms = get_terms($taxonomies, $args);

  $return .= '<ul>'; 

    foreach ( $terms as $term ) {

      // return terms (working)
      $return .= sprintf(
        '<li id="category-%1$s" class="toggle">%2$s <span class="cat-description">%3$s</span>',       
        $term->term_id,
        $term->name,
        $term->description
      );

        $subterms = get_terms($taxonomies, array(
          'parent'   => $term->term_id,
          'hide_empty' => false
          ));

        $return .= '<ul>';

        foreach ( $subterms as $subterm ) {

          //return sub terms (not working ??? )
          $return .= sprintf(
          '<li id="category-%1$s" class="toggle">%2$s <span class="cat-description">%3$s</span>',       
          $subterm->term_id,
          $subterm->name,
          $subterm->description
          );

          $return .= '</li>'; //end subterms li
        }            

        $return .= '</ul>'; //end subterms ul

      $return .= '</li>'; //end terms li
    } //end foreach term

  $return .= '</ul>';

return $return;
}