What's the Name of That Book??? discussion

I can't remember author, publisher, or cover, but was probably published between 1985-1995.

A cylinder enters Earth's solar system (near Saturn?), open at one end. A mission is sent to investigate. Upon entering the cylinder it is found to go on and on infinitely, although the outside is only X meters long. The inside is nondescript and deserted. A probe is sent down the tunnel, but after several hundred/ thousand? kilometers still nothing is found, but there are starting to be irregularities in the tunnel's surface.

Tens of thousands of kilometers down the tunnel there are people and cities. The tunnel that far down had started destabilizing and cracks were appearing. While I think these people were human, they had avatars or genetic engineering which allowed them to basically take on any shape they wanted. I also think that something was coming through the cracks / holes in the tunnel.

I don't remember the conclusion.

Any help would be greatly appreciated.

Cheers!

:)

Thanks! But now they're going to get harder as my memory fades...

lol...I have a few ones still not figured out, too