The Visceral Heap, formerly the Bloody Heap, is a Nether biome added in 1.6.4 shortly after Biomes O' Plenty's original release. The floor is made of Flesh with some Eyebulb flowers and pools of Liquid Blood
First of all, you shouldn't be attempting to survive here, unless you are an incredibly experienced player with good knowledge of the Nether. But if you must know, Flesh makes a great building block. The Nether is not the place to be surviving, so make sure you bring a lot of cobblestone to block out those ghast's blasts.
As of early April, 2016, Nether Biomes have not be re-added to 1.9 and 1.8.9 BoP, though the programmers are planning to bring them back. They are in 1.7.10 and earlier, though.
However, in version 1.11.2+ all nether biomes except Boneyard are readded to BOP.