Lime Kilns in the Santa Cruz Mountains, which made lime by heating limestone at 900 deg C (CaCO3 + heat → CaO + CO2), burning redwoods with the stone in the sealed chambers (kilns). This site was active from the 1870s to 1919 when most of the redwoods had been denuded from the area.
Ugly grates cover the kiln openings. You can go over the top, but there isn't too much to see inside.