There is only one place left to investigate massive mass outbreaks: the Alabaster Icelands.

The Request

After you complete Massive Mass Outbreak in the Highlands, talk to Mai in Jubilife Village near the Front Gate to accept this request.

Investigate Massive Mass Outbreaks

Talk to Mai if you want to feed Munchlax some Aguav Berries to reveal the massive mass outbreaks on the map. As before, catch or defeat all of the pokémon at a massive mass outbreak to clear it. Repeat this until you have cleared three of them. If you take too long and the massive mass outbreak ends, go back to Jubilife Village, then go to any area outside of the village, and keep going back and forth until another massive mass outbreak occurs.

Report to Mai

Afterward, go to a base camp and talk to Mai to proceed. After you receive your reward (listed below), you will automatically report any caught pokémon and observations, then will return to Jubilife Village.

Reward

You get 30 Iron Chunks, 20 Caster Ferns, and 20 Balls of Mud.