Nature Nanotechnology, Published online: 28 September 2026; doi:10.1038/s41565-026-02284-0MRI-responsive hydrogel microparticles embedded with zinc ferrite nanoparticles enable deep-tissue mapping of dynamic tumour compression, revealing that transient stress surges, rather than cumulative mechanical load, lead to the epithelial–mesenchymal transition and tumour remodelling.