The Mumm-Ra's Grotto was a makeshift location of operation for Mumm-Ra after the first Black Pyramid on New Thundera was destroyed. After a short time, he'd return to New Thundera and have built a second proper pyramid, which made this location obsolete.
When Mumm-Ra was assigned by the Ancient Spirits of Evil the task of finding the treasures of Thundera, he needed a place to stay on New Thundera, so when he came there, he found a cavern near his old Black Pyramid's ruins and converted it to suit his needs. It appears to have been retired as a location for Mumm-Ra, either when the Ancient Spirits of Evil locked him away after failing on the last day or retired due to wanting to build a more established location which renders to Grotto, redundant.
Located in a cavern near the ruins of the previous Black Pyramid on New Thundera, it was hid from the ThunderCats, allowing for the element of surprise. The same ruins near the old pyramid are still around above on the ground, while the cavern appears to have another opening going elsewhere, though never shown utilized.
Much like the pyramids, inside it was pretty much like the throne room, housing: his sarcophagus, magic cauldron (hexagon in design, instead of circle), the Ancient Spirits of Evil statues, and the Mumm-Raft.
- Black Pyramid (Third Earth)
- Mumm-Raft (travelling pyramid)
- Black Pyramid (first on New Thundera)
- Black Pyramid (second on New Thundera)