This article focuses on the autonomous exploration of unknown underground environments using a swarm of drones. In such settings, conventional wireless communication is often unavailable; therefore, we deploy a chain of visually connected UAVs that maintain and reorganize themselves using only local vision-based communications. The proposed algorithm is decentralized and based on two distinct roles of the agents: the 'leader" performs the exploration, and the 'followers' preserve connectivity between the Leader and an operator at the entrance. We evaluate the proposed approach across various scenarios using a realistic simulation tool that incorporates a UAV dynamics model.