Structural lore — the two windless oceanic dead-zones flanking the Grand Line on both sides.
What we know
Ch. 101 pages 6-7: there are two Calm Belts, one on each side (north and south) of the Grand Line — ‘the Grand Line is sandwiched between two other seas’ (Nami). The Calm Belts are windless — no wind blows.
Ch. 101 page 6: the boundary between the storm-system around the Grand Line and the Calm Belt is abrupt — the storm stops dead at the Calm-Belt edge (‘the storm’s still raging over there… but there’s not even a breeze here’).
Ch. 101 pages 8-10: the Calm Belt is a Sea-King nest — colonies of giant Neptunians live in the windless water. The Sea-Kings are huge, plural, and nesting (per Sanji’s it’s a nest line on page 10).
Ch. 101: Calm-Belt sailing is operationally suicidal for a wind-sail vessel — the Going Merry has to row out (sails useless) and the Neptunian-encounter is potentially-lethal.
Ch. 488 (p. 5): Yorki, captain of the Rumbar Pirates, split from the crew and sailed out through the Calm Belt ~50 years ago — “I wonder if Captain Yorki made it safely out of the Calm Belt?” An incidental data point that the Calm Belt was (perilously) used as an exit route from the Grand Line by this crew; his survival is left unknown. See Calico Yorki’s Unknown Fate After Splitting From The Rumbar Pirates.
Chronology / structure
The Calm Belts are permanent structural features of the world’s hydrography, not weather events. They flank the Grand Line as parallel bands on the north and south.
Open questions about this topic
How do the Marines / Celestial Dragons cross the Calm Belt?.
How does Boa Hancock cross?.
Are there Calm-Belt-fish-and-flora distinct from open-ocean species?.