UPDATE: They have just announced the extension of the Colosseum underground tours (hypogeum, dungeons, subterranean levels, whatever you want to call them!) along with the third level. It is now officially confirmed to be opened until July 31st. You can continue booking with Pierreci through their phone system:

Pierreci Booking Info:
http://www.pierreci.it/en/museums-an.../colosseo.aspx

Keep in mind that will book you on a tour with 25 people and a colosseum tour guide that does the same tour all day long. Furthermore, here is information on how to book directly online with a company called Walks of Italy, and they get to use their own, fluent English speaking guides, and will visit the Roman Forum as well, with a maximum of 12 per group. This is brand new since before you always had to use Colosseum guides.

Walks of Italy's Blog Post:
http://www.walksofitaly.com/blog/rom...-rome-hypogeum

Hope that's helpful!