Kohtla-Järve is a city and municipality in north-eastern Estonia, founded in 1924 and incorporated as a town in 1946. The city is highly industrial, and both processes oil shales and is a large producer of various petroleum products. The city is also very diverse ethnically: it contains people of nearly 40 ethnic groups[citation needed] and only 21% of the population are Estonians. Many ethnic Estonians in the city have adopted Russian as their primary language, although this changed in the late 1990s, as Estonian once again became the primary language used in everyday life. Kohtla-Järve is the 4th largest city in Estonia.