OK, lets estimate how much concrete and gravel you need.
Have a look at this thickened slab detail:
http://www.drawings911.com/product_images/m/footingmono_12__52585.jpg
The edges of your floor pad need to be 16x16 for your 8 block. This is the footer. It needs to support the walls, roof/ceiling and 25 tons of dirt above (if only 3 below the surface) plus the weight of the greenhouse and whatever else is above. Using this calculator (EDIT:
http://www.concrete.com/calculators/concrete-materials-calculators) Ive estimated the footer needs 4.4 cu yds. The pad (16x18 to allow for a 12x14 interior) only needs to be 4 thick cause its not supporting much. You only need 3.5 cu yds for the pad. So for the footer and the pad you need nearly 8 cu yds. A concrete truck holds between 8-9 yards. You can pour the footer and pad in one shot. Also you need an equal about of 2B gravel under all concrete. So 8 cu yds of gravel. One cu yd of gravel weighs over 3000lbs. Youll need to clear a road for your stone, concrete and block deliveries.
This is just the footer and floor slab. Youll probably need another 8 yards of concrete for the ceiling slab. No gravel for this though, you set corrugated steel pan on top of your block walls and this supports the ceiling slab. Honestly I cant say with any certainty that a 6 ceiling slab wont cave in. Youll probably need some lolly columns in the middle of the room. This is why you need an engineer.
Youll probably need another gravel delivery and concrete truck for the tunnel too. So all in all 3 pours. You have to wait 28 days before building anything on the concrete unless you get high early strength concrete. The average price of concrete is $125 per yard delivered. Its way more if you buy it by the bag from lowes and its not really practical for a job this big. So 24 yards of concrete youre talking $3,000. This doesnt include your gravel, rebar, corrugated steel pan or welded wire mesh. Dont forget all the wood for formwork. 8 Concrete block, mortar, and rebar and grout for the cores (gives lateral strength so the wall doesnt collapse when you back fill dirt against it) over $2,000.
This whole build will easily cost over 10 grand in materials alone. Doing all the work yourself, it will take months. If I were building this for a customer Id tell them right off the bat youre looking at $30,000 and 3 months with a crew of 6 guys.
Reinforcing a shipping container would be much easier and cheaper. But youll need to rent a crane to place it. With a project this big, I can absolutely guarantee the building inspector or perhaps even OSHA will come by. Silly question, but do you own the land where this project is going?