Strictly speaking, you can't put a <ul> directly in a <ul> --- it has to be inside one of the <li>'s, like this:
<ul>
  <li>First major
    <ul>
      <li>First minor</li>
      <li>Second minor</li>
    </ul>
  </li>
  <li>Second major
    <ul>
      <li>Third minor</li>
    </ul>
  </li>
</ul>