The effectiveness of monitoring and early-warning systems for ground deformation phenomena, such as sinkholes, depends on their ability to accurately resolve the ongoing ground displacement and detect the subtle deformation preceding catastrophic failures. Sagging sinkholes with a slow subsidence rate and diffuse edges pose a significant challenge for subsidence monitoring due to the low deformation rates and limited lateral strain gradients. In this work, we satisfactorily illustrate the practicality of the Brillouin optical time domain analysis (BOTDA) to measure the spatial-temporal patterns of the vertical displacement in such challenging slow-moving sagging sinkholes. To assess the performance of the approach, we compare the strain recorded by the distributed optical fiber sensor with the vertical displacement measured by high-precision leveling. The results show a good spatial correlation with the ability to identify the maximum subsidence point. There is also a good temporal correlation with the detection of an acceleration phase in the subsidence associated with a flood event.