How were Finnish hiisi and Estonian hiis sacred groves used?

The Finnish hiisi and Estonian hiis were sacred groves that served as significant religious sites among Finno-Ugric peoples. While historical documentation regarding the specific rituals performed within these groves is limited, they are considered comparable to other traditional sacrificial sites, such as the lud or keremet, where communities gathered for religious observances.1

In similar Finno-Ugric traditions, these groves functioned as hallowed spaces where sacrifices were offered to deities and spirits. For instance, in Ingria, sacred groves remained in use through the late 19th century for directing prayers and offerings to figures such as the thunder god Ukko and the vegetation deity Sämpsä. These sites were generally treated with extreme reverence, often requiring participants to maintain ritual purity, such as bathing before entry, and prohibiting unseemly behavior within the enclosure.1