The paper discusses the results of an experimental study on combustion development in a large volume filled with ultra-lean hydrogen-air mixture under terrestrial gravity conditions. For the first time, characteristicflame ball evolutionIn particular, ittheBesides, the lateral expansion of the flame ball plays an important role, defining the changes in curvature radius and stretching of the flame. Due to this, flame breaks up and secondary flame kernels arise. So, the developed structure of the ultra-lean flame occurs to be much more complex than earlier registered cap-shaped flames. Large-scale dynamics of the flame ball is of paramount interest for understanding the stability of near-limit flames and for estimation of hazards related to the development of flame balls under real conditions. Studied processes represent a possible way of energy transfer from the spatial areas filled with less reactive mixtures toward areas with high mixture reactivity. Such a phenomenon stages of the during its convective upward motion on large spatial scales are described. The physical mechanisms defining the particularities of ultra-lean flame evolution subjected to the natural convection are proposed. is shown that the key role in the process of flame propagation in the ultra-lean mixture belongs to the convective rise of the hot combustion products in the gravity field. Herewith, velocity of the flame upward motion occurs to be much higher than the laminar burning velocity in the considered ultra-lean hydrogen-air mixture. may become a reason for the development of emergencies and should be thoroughly investigated and taken into account while designing reliable fire and explosion safety systems. Experimental results are compared with previously obtained numerical data that allowed to demonstrate the correctness of the mathematical modeling and physical mechanisms identified on its basis.