How to Create Nested Submenus

I am trying to create a main menu that contains multiple layers of Submenus but cannot figure out how to set it up.

For example, by hovering over the first element a submenu will appear with more hover elements that will cause another submenu to appear.

When I attach a submenu into another submenu I am unable to add any content to the nested submenu.

Any help with this would be greatly appreciated.

currently mega-menu is under the development, we got a feature request for it, but it is possible with Dev Mode