I'm struggling with an issue with my drop-down menu, which is that the dropdown menu is displayed properly, but disappears once it stretches below the containing div. Here's an illustration of what i'm talking about: 

![dropdown menu](https://i.stack.imgur.com/2QfrA.png) 

The menu only extends to the height of the black containing `<div>`. Here's my framework: 

```
<ul>
     <li><a href=''>Menu Item</a></li>
     <li><a href=''>Menu Item 2</a>
     <ul class="sub_menu">
          <li><a href=''>...</a></li>
          <li><a href=''>...</a></li>
          <li><a href=''>...</a></li>
     </ul>
</ul>

```