Question: How Thick Is The Flat Earth? What Is On The Bottom?

Answer 

Flat Earth Theory (FET) does not pretend to fully understand physical realities that have not been directly observed. That being said, the Flat Earth does possess a certain size and shape. The most widespread assumption holds that Earth is a cylindrical planetary body with a Flat, upwards facing surface upon which humanity resides. Sperists measures of the ball Earth place its radius at 4000miles. And as this graphic suggests, even mainstream science admits that knowledge of Earth's interior is foggy at best: 

http://www.livescience.com/6959-hole-drilled-bottom-earth-crust-breakthrough-mantle-looms.html

That being said, Flat Earth Theory can offer an educated (though empirically uninformed) guess regarding the dimensions of the Flat Earth. Lets start with the dimensions of the surface. While spherist measurements of Earth's interior commit glaring errors, the measurements themselves represent data that Flat Earth Theorists can utilize with cautionary assumptions in mind. (Please remember that science itself is a corrupted methodology, and that any data appropriated must be done so with caution). It is a common fact of empirical observation that earth contains a supper heated molten interior, as evidenced by volcanoes and other geothermic activity. A significant and sizable outer crust necessarily exists in order to hold back such volatile fluids. The molten layer must therefore be sandwiched between two significantly thick layers of crust. As mentioned, spherist measurements place the radius of Earth at approximately 4000 miles . 

Taking the above measurement as a tentative starting point, an educated estimate of the Flat Earth's proximal (upper; above the molten layer) crust places it at approximately 4000-6000 miles in thickness. The molten layer likely extends anywhere from 1000-2000 miles in depth. The thickness of the Flat Earth's distal (lower; below the molten layer) crust likely corresponds to the thickness of the proximal crust, ranging between 4000-6000 miles. This places the Flat Earth's entire thickness from top to bottom anywhere from 9,000-14,000 in length. Please note that these are only estimates, and that the spherist data appropriated is suspect. However, short of digging through the Earth itself, empirical descriptions of the Earth's interior (Flat or spherical) will remain subjective at best...

So what is at the bottom of the Flat Earth? 

The bottom of the Flat Earth almost certainly consists of the same material composing the upward facing surface. (Remember, contrary to spherist belief there is no reason to assume that up or down does not exist in space. If the universe is finite, thus space almost certainly contains a downward direction towards which all unsupported objects would tend to fall toward). As any material loosely adhering to the bottom of the Flat Earth would have fallen off long ago, the bottom likely displays a coarse surface composed of rock and ice, possibly taking the form of icicles and stalactites. The roof of a cave could resemble the bottom of the Earth, but on a much larger scale...