Coastal areas surrounding Tokyo are expecting the arrival of a mass of floating pumice stones, created by a huge underwater volcanic eruption in August. The material has already blanketed dozens of fishing ports and impacted marine life in southwestern Japan.

According to a supercomputer-assisted forecast by the Japan Agency for Marine-Earth Science and Technology, the volcanic stones are expected to reach waters off the coasts of Chiba and Kanagawa prefectures in late November.

The stones are believed to have originated from the Fukutoku-Okanoba underwater volcano, located in the Ogasawara island chain in the Pacific Ocean. It erupted on Aug. 13 in one of the largest volcanic events seen anywhere in modern-day Japan.