Two memorials have been left in Kevin the Cube's memory. One of them has to be found to complete one of "Worlds Collide" mission challenges. Visiting two in one single game would allow you to complete the prestige version of the challenge. Here are their locations:
The first memorial is in the desert, on the hill hit by a series of lightning strikes. This is where Kevin first appeared on the map.
The second one is at Loot Lake, where Kevin disappeared. The memorial is located on a small island in the lower left part of Loot Lake on the map.
Feel free to check out our other guides for Worlds Collide challenges to complete them all as quickly as possible.