The 3DMark Fire Strike Ultra Combined Test stresses both the GPU and CPU simultaneously. The GPU load combines elements from Graphics Test 1 and 2 using tessellation, volumetric illumination, fluid simulation, particle simulation, FFT-based bloom, and depth of field.
The CPU load comes from the rigid body physics of the breaking statues in the background. There are 32 simulation worlds running in separate threads each containing one statue decomposing into 113 parts. Additionally, there are 16 invisible rigid bodies in each world except the one closest to the camera to push the decomposed elements apart. The simulations run on one thread per available CPU core.
The 3DMark Fire Strike Utra Combined Test uses the Bullet Open Source Physics Library.