Mapping is an essential task in many robotics applications. To build a map, it is frequently assumed that the positions of the robots are a priori unknown and need to be estimated during operation. Multi-robot SLAM is a research direction that addresses the collective exploration and mapping of unknown environments by multi-robot systems. Yet, most results so far have been achieved for small groups of robots. Multi-robot SLAM is still a growing field, and a number of research directions are yet to be explored. Among them, swarm SLAM is an alternative, promising approach that takes advantage of the characteristics of robot swarms.