In this question, there is an image of a ladder leading to some hole in the bottom of the nose of an A330. What is that hole for?
It is the avionic compartment access door as described in the Flight Crew Operating Manual!
An inward-opening, manually operated, hinged door gives access to the avionics compartment. This door is in the lower fuselage, forward of the nose landing gear bay. A ladder is stowed inside the compartment adjacent to this door, which may be either be operated from the interior or the exterior. This compartment is also accessible from the cockpit, via a floor hatch located behind the Captains' seat. A fixed ladder in the avionics compartment for access from the cockpit.
Here is a drawing from the outside!
And here a view from the cockpit flight deck for an A340!
That 'hole' gives access to the avionics comparment- forward zone. The comparment has a ground access ladder, which is seen in the image. The deployment of that ladder (which is similar in A330/A340) can be seen in this video.