Krana tombs?

In the Bohrok Animations, after the defeat of the Bohrok, we see a the Matoran throwing the Krana into a pit near Ga-Koro and sealing them away with a boulder. We later see this same tomb in MNOG II. Were there more tombs made across the island to seal the Krana away? And were these tombs made by the Matoran, or did they just find a random pit and threw them down there?