I remember when the game first launched, there used to be a way to fly to a Borg Unicomplex and fight them on their own turf. I remember that the objective was to disable certain nodes and such while fighting off cubes and spheres. Part of the difficulty that I always remembered was that it was often one ship vs. at least 2 cubes and 2 spheres at any given area of the map, and they weren't afraid to use a tractor beam on you the first chance they had.

They've obviously been taken out since then, but whatever happened to it?

It's still there. The system you're looking for is called Ker'rat, which is in the Eta Eridani sector. It's a Space War Zone.
http://www.stowiki.org/Ker%27rat_System

I hope this helps!

Ahh, I thought the Ker'rat system was a PVP zone, which I don't participate in. I only experienced way back 2 years ago when I had a free 5 day trial.

Its a PvEvP zone...

2 years ago, there was very little chance to run into each other in Ker'rat because there were very few klingons, but now it seems to be evenly matched.