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Danny's mountain-spanning gym on Navel Island — second stop on the Orange League circuit, awarding the Sea Ruby Badge.
Navel Gym's challenge takes the better part of a day. A challenger climbs the mountain using only Pokémon moves (no gear), freezes a summit lake with an Ice Pokémon, builds a makeshift sled from the grove, races it down the mountain to the beach below, and only then battles Danny three-on-three on the sand. It is the most physically demanding gym in the circuit.
Not a single building but the entire Navel mountain: a climbing course up bamboo-strewn slopes, a frozen lake at the summit, a bamboo grove for sled crafting, and a beach-arena battlefield at the foot where the final Pokémon match is fought.