What was polly and digory's motive for exploring the tunnels that linked the house together?

First, they were not tunnels in the sense of being underground. The houses were linked together by attics that, taken together, resembled tunnels. (Think of English row houses with common walls and one large attic connecting all the houses below.) The two children wanted to find out why one of the houses was empty and see what was in it, but apparently made a mistake in calculating how far they should walk and came back into Digory's house instead of the one they wanted.