Greta Thunberg has announced a surprise visit to Rapid City on Monday.

The 16-year-old Swedish climate activist made the announcement in Pine Ridge on Sunday where she took part in a panel aimed at tackling environmental and Indigenous issues.

Thunberg met with fellow activist Tokata Iron Eyes and other youth leaders from the Red Cloud Indian School.

Their friendship began in September when they met at George Washington University.

Thunberg accepted Tokata Iron Eyes' invitation to visit South Dakota and address the two tribal nations.

Together Thunberg and Tokata Iron Eyes announced a plan to march on the mayor's office in Rapid City.

They plan to meet at Memorial Park after 11 a.m. on Monday before, presumably, heading to the city hall.

Fossil fuel use is one of the many topics they said they plan to bring forward to the mayor.

Thunberg is from Stockholm, Sweden and gained international attention by delivering a scolding speech to world leaders at the United Nations.

She organized a worldwide school climate strike back in September that drew millions of people to the streets in more than a dozen countries.

Tokata Iron Eyes is from Standing Rock Nation and now lives at Pine Ridge.

She is an Indigenous and environmental justice advocate who protested the Dakota Access pipeline.