You could create a procedural generated tunnel system where you create tunnel modules the length of say 2000 units long in 3dsmax or whatever modelling program you have. You create 5-10 modules. Then you program so that you generate 4 tunnels ahead of player position and once hes in a new tunnel module you generate a new one 4 modules ahead of him. once the 20th or what ever you choose tunnel has been entered you can then spawn the beginning tunnel as the new tunnel which in turns makes it seem like hes back at the beginning. If you want to have it loop forever you can save what modules are used for each tunnel system in an array.
That is what i would do and hopefully that makes sense.
ZZ