Ranu Kumbolo, Mountain lake in East Java, Indonesia

Ranu Kumbolo is a volcanic maar lake in the Jambangan caldera of Bromo Tengger Semeru National Park, in Pasrujambe District, Lumajang Regency, East Java. It covers 12 hectares, making it the largest lake in the park, and lies at 2,395 metres on the Ranu Pani–Semeru trail. Its dark green water fills a basin between grassy banks and forested slopes.

An andesite stone inscription found near the lake, the Prasasti Ranu Kumbolo, bears one sentence in Old Javanese dated to 1447 CE. It records the visit of Mpu Kameswara, who came to perform a tirthayatra ceremony. The find reinforced the Tengger community's tradition that the lake's water is sacred and purifying.

This lake serves as a gathering place where mountain climbers from different routes meet and mingle during their journeys. It holds meaning for the local communities living in the surrounding villages who depend on these waters.

The area requires permits from the ranger office and warm clothing due to very cool conditions. Visitors should arrive early to experience better light and less crowded conditions.

Near the lake stands the Ranu Kumbolo inscription, a single sentence in Old Javanese carved on an andesite stone and dated to about 1447 CE. It records that Mpu Kameswara came here to perform a tirthayatra, a pilgrimage ritual, which reinforced the Tengger tradition that treats the lake's water as sacred.