Once discarded into marine environments, plastics start to degrade until complete mineralization. For polyethylene, it is estimated that this process takes about 1.4 to 1,500 years. The high uncertainty comes from the lack of long-term investigations under environmentally relevant conditions, as information on the age of environmental plastics is typically not available. One exceptional case are microplastic resin pellets from the SS Hamada, whose age is exactly dated by the day of foundering twenty-nine years ago in the Wadi el Gemal national park, Egypt. Its plastic load made of low-density polyethylene ended up in two in situ laboratories: a beach, characterized by high temperatures, heavy UV radiation, waves, and tidal range, plus the dark and cold saltwater inside the cargo holds at 18 meters depth. For these very different sample sites, the status quo of plastic degradation was comparatively assessed by imaging, chemical, and rheological methods. We found that even after nearly three decades, microplastics in the Red Sea were in a pristine-like state, whereas microplastics on the beach have been degraded on all structural levels. In particular, a distinct crack network, an increased number of carbonyl groups, cross-linking of chain branches, a higher crystallinity, and an altered rheological behavior was observed, all of which resulted in fragmentation. Altogether, this proves that plastic degradation is considerably slower in the ocean compared to beaches. Here, photochemical and thermo-oxidative weathering accelerate the degradation as a consequence of the abiotic conditions, which the beached microplastic resin pellets from the SS Hamada have to face. With this study object on hand, future monitoring should enable the calibration of existing models to derive robust estimates for degradation rates of plastics in different marine environments.