Seismic observations have revealed two Large Low Shear Velocity Provinces (LLSVPs) beneath Africa and the Pacific Ocean in the lowermost mantle1–4. However, the detailed morphology and stability of the LLSVPs and their interaction with the surrounding mantle remain unclear. Here we show that, whereas the maximum height of the Pacific LLSVP is ~500 – 700 km, the African LLSVP reaches a height of ~1,500 – 1,800 km from the core-mantle boundary. With numerical modeling experiments, we find that the height of the Pacific LLSVP can be reconciled by a wide range of the LLSVP density and surrounding mantle viscosity. However, the large height of the African LLSVP indicates that it may be less dense and thus less stable than the Pacific LLSVP.