What is the mood or atmosphere?

For the most part, the mood and atmosphere of the novel is dark and forboding, although, there are intermittent moments of joy and humor.

There were many paths that led up into those mountains, and many passes over them. But most of the paths were cheats and deceptions and led nowhere or to bad ends; and most of the passes were infested by evil things and dreadful dangers.

Not far ahead were dreary hills, rising higher and higher, dark with trees.

Up the hill they went; but there was no proper path to be seen, such as might lead to a house or a farm; and do what they could they made a deal of rustling and crackling and creaking (and a good deal of grumbling and drafting), as they went through the trees in the pitch dark.

The dwarves were still passing the cup from hand to hand and talking delightedly of the recovery of their treasure, when suddenly a vast rumbling woke in the mountain underneath as if it was an old volcano that had made up its mind to start eruptions once again.

How does bilbo make an effort to start the search for the secret door