ok, if the ships are able to meet the machines and try a sneak attack, why would the machines need to dig anymore, or why did they need to dig at all. they could take the routes that the ships took to get to the surface, the ships are much larger than the sentinels. and they obviously know where zion is as they have destroyed it several times and know exactly where to dig.

is it to give the people hope that zion can stop them or is there some reason i'm missing?

I don't think it was really explained, but there could be several reasons.

1) The squiddies don't know where the tunnels back to Zion are
2) The routes are booby trapped with EMP mines
3) I can't think of another, but I'm sure there is a 3.

The diggers and sentiels(sp) are digging through the back way. the tunnels back are filled with traps, guards (in those machines) and a whole mess of other items.

Why fight all that when you could just dig around it to get to them as they have no defense completely around zion where they assumed the walls were the places they didn't have to secure.

makes sense, they could have mentioned it somewhere

But why risk losing all their ships. Without ships they can't get to broadcast depth and enter The Matrix. Nor can they get to the canal dumping zone to retrieve freed humans.

Tell that to Lock. it was his idea.

They sort of did. When Morpheus' ship docks in Zion for the first time there's a shot of the ship coming in through a tunnel, and, in the foreground, what I thought looked like an automated gun turns away from aiming at the ship. I just took that to mean that the tunnels were well protected.