I put it in the same category as the crossovers at the Rutland Halloween parade.
This is what MCS says about it;
Aquaman #56 and Sub-Mariner #72 is the infamous hidden DC/Marvel crossover! Writer Steve Skeates, unbeknownst to the editorial staffs of Marvel and DC, creates the very first intercompany crossover between the Big Two. Aquaman battles a creature that at the end of the issue explodes in outer space. A few years later, the very same creature emerges in the Marvel Universe from an explosion in outer space. This has to be one of the most underrated and coolest hidden gems in Comics History. – Odinson, Lone Star Comics Archivist Cover price $0.15.