It is a 100 gal divided lengthwise. The animals are in the narrow part 5 feet long and 6 inches wide. the bottom of the burrow is a 3.5 foot piece of 4 inch thin walled pvc with 45 degree elbows on each end. I started the animals with only a few inches of sand in that section of the tank. As they started mixing the sand with mucus and buiding a burrow around the entrance of the pvc, I gradually added more sand to the point that it is now about 16 inches deep and the animals have natural sand burrows on both ends. If you want to see what the animals are doing, you can slice off a thin strip of the pvc and seal it to the side wall with silicone and cover it with black plastic on the outside of the tank. The animals will build the vertical portions of their burrow bending away from the glass, but you can still see what is going on in the bottom of the burrow.
If you want to get really fancy, you can use the cut away burrow technique, but I placed a sheet of Polaroid on the inside of the tank wall covering the cut away section of the burrow. On the outside of the tank I have a second set of Polaroid panels that I can rotate to completely block light from the burrow or allow as much as desired to be transmitted. This allows us to see brooding, molting, watch the male feed the female, etc.
Roy