Résumé : Like Galileo turning his telescope to the sky's immensity, discovering planets and stars, the seismologist dreams of viewing through the Earth, exploring its structure, and focusing on small-scale features to unveil their nature and origin. The lowermost part of the Earth's mantle, halfway to its center above the liquid core, is particularly mysterious and exhibits remarkable yet poorly understood seismological features. The most recent tomographic images or models unveil Large Low Shear Velocity Provinces (LLSVPs), which are crucial to understanding past and present Earth's mantle circulation, chemical composition, and thermal structure. There is a need to sharpen our tomographic models that lack resolution for addressing such fundamental issues, bringing new insights into these mysterious structures. A striking observation is a presence of 'Ultra-Low Velocity Zones' (ULVZ) toward the edges of the LLSVPs, near the root of hot plumes rising through the mantle and manifesting as volcanic activity at the surface. ULVZs are small objects, at most tens of kilometers high, well below the resolution limit of current tomographic models. 3D imaging of these extreme heterogeneities' precise contours is a considerable endeavor and has yet to be attempted. Recent theoretical and methodological advances allow us to meet this challenge. We developed Box-Tomography, which makes it possible, for the first time, to obtain localized high-resolution tomographic images at arbitrary locations in the deep Earth where no seismic sources nor receivers are present. We will present our latest methodological developments in forward and inverse modeling and their relevance for investing in the deep Earth's structure at the core-mantle boundary.