Great thread and huge congrats on the find. If nothing else for the historical and coolness factor. I have a VERY important tip for you... cover that freaking doorway ASAP. Keep it secret, keep it safe. The last thing you need is someone unexpectedly finding the entrance and blowing the entire operation. Trust me. The greatest value in having a secret grow room is it remaining just that... a secret grow room. Tell us, preferably often and with plenty of pics! But some random HVAC tech or plumber would love to tell his buddies about the crazy underground room he saw at your house. I know you don't plan on letting anyone down there and only family will see it but, uh... things happen. And once the cat is out of the bag it is impossible to shove it back in.
Get a large locking metal cabinet, secure it to the wall, and cut out the back of the cabinet. Second, as amazing as this find is you have to keep in mind that cleaning it up, making it safe, dealing with water seepage, and transforming it into any kind of habitable/workable space is going to be a MASSIVE PROJECT. I'm talking a metric crap ton of man hours here and you can't just call all your friends in to help for pizza and beer. If you have a grow buddy you absolutely trust, then you just cut your work in half. But doing this on your own will take a ridiculous amount of man hours and logistics (not to mention a fair amount of cash.) Still, it would most likely be worth it. Everyone on here will be cheering you the entire way. Maybe I missed it but do you own the house? Do you have any plans on moving in the near or far future?