Microplastics permeate the ocean water column, yet our understanding remains strongly biased toward the surface because most surveys rely on net tows in the upper tens of centimetres. Global observations show that reported abundances span eight orders of magnitude, with particle size emerging as a primary control. Small microplastics (1–100 μm) decline gradually with depth, suggesting prolonged residence and a comparatively even distribution, whereas larger particles (100–5,000 μm) are more strongly retained near density-stratified layers and within the upper 100 m, or are rapidly exported toward the deep ocean. Large-volume in situ filtration detected microplastics from subsurface waters to 4,000 m and yielded substantially lower and more representative abundance estimates than small-volume CTD sampling. Similarities in polymer and shape composition between surface and interior waters, together with the widespread occurrence of PET fibres and PP/PET particles at depth, support sustained downward transfer from the surface and lateral redistribution by ocean currents. These findings indicate that the ocean interior is not a passive sink,  but a dynamic reservoir in which microplastics interact with stratification, eddy subduction, marine aggregates, biofilms and biological particle fluxes. However, inconsistencies in sampling volume, mesh size, analytical detection limits and reporting units continue to constrain global synthesis. We therefore propose a coordinated framework combining standardized large-volume, depth-resolved sampling, continuous particle characterization, long-term observations and coupled physical–biogeochemical modelling to better constrain the oceanic plastic budget, identify subsurface accumulation zones and assess ecological and biogeochemical impacts.