There have been a range of underfloor accommodations in widebody aircraft over the years, strangely none of them seem to have lasted.

The Ilyushin 86 does indeed have lower level airstairs and you can carry your baggage on board, leave it in storage areas, then go up internal stairs to the cabin. It takes a long time to get off with people gettuing their bags, but then no waits in the terminal. It looked an excellent idea, making the aircraft quite independent of ground equipment at remote airports, but never caught on elsewhere.

A few airlines specified Tristars and DC-10s with underfloor galleys which are accessed by rather small elevators (also used as dumbwaiters to carry the food up) but like many underfloor areas they were not certified to be occupied on takeoff/landing. As a result there needs to be an emergency ladder route in case the elevator fails in flight when crew are below.