Retrieving the Mysterious Box is the main goal of the "Find the Mysterious Box" quest, which is randomly offered to the player at port in London.
A peculiar scientist asks the explorer to bring him the box back, after the expedition. If player agrees to deliver, he will receive the Scientist's Map.
Digging in a spot marked on the map (by standing on marked hex and using a shovel) will allow party to loot the box.
Opening the Box
Instead of being brought back to the scientist, the mysterious box can be opened during expedition. If opened, the explorers can see that inside the box there is a dead cat. Suddenly, a small black spot appears and the void, a dark and impenetrable shadow, starts devouring everything in its path. If the expedition doesn't run away fast enough, they will be trapped by the void and die.
This is a direct reference to the thought experiment "Schrödinger's Cat".