I would also recommend a hay substrate (I use a mix of orchard grass, which is quite soft, and timothy hay).
Under the hay, though, I would avoid anything absorbative as it may serve as a petri dish and a possible vector for disease (especially if mixing species). I use a tarp bottom. It means the hay gets soiled quicker, but as long as you are regular with cleaning, it shouldn't be a problem.
If you are sitting them on the ground of the basement, a layer of cushioning (such as used carpeting) under the tarp is a good idea, for both comfort, and insulation.
>>I am making a new pen for my tortoises in my basement for the winter time. the pen is basically an 8' X 8' box. I have two young sulcatas and one russian tortoise who acctually all get along very well. I have heard of using timothy hay as a substrate but im not sure they will like only that and i dont really want to have to carry 30 bags of dirt into my basement but i will if i have to. anyone got any ideas??Thanks a lot
>> ~Ryan