You see this in movies all of the time. Someone has hidden a bomb, or snakes or terrorists or something in the cargo hold and the hero opens a hatch and climbs down to take care of it. Sometimes there is even an elevator you can take to get there.
Yet you never see these hatches in real life, at least not in my experience. And it would seem like a great security risk to have easy access to all the stuff that they wouldn’t let you carry on as cabin luggage.