Mitiaro's interior is dominated by two freshwater lakes: Te Rotonui (Big Lake) and Te Rotoiti (Small Lake), which together cover a substantial portion of the island's total area. The lakes are bracketed by the encircling makatea and the central taro swamp plateau, giving Mitiaro a geography found nowhere else in the Cook Islands.
Both lakes are home to itiki (freshwater eels), which feature in local cuisine, and to populations of milkfish (a'va). The surrounding wetlands and the lakeshores attract migratory and resident birds, including species rarely seen elsewhere in the archipelago. For birdwatchers, a quiet morning on the lakeside is excellent: the light is good early and the birds — including reed warblers and kingfishers — are active before the heat sets in. The lakes can be reached on foot from Mangarei village.