The Département de Mayotte is is an overseas department and region and collectivité territoriale unique (single territorial collectivity) of France. In other words, the group of islands, located north of Madagaskar, has the powers of both a region and a department, which provides for local autonomy within limits prescribed by law. The Indian Ocean between Northwestern Madagascar and Northeastern Mozambique is also called the Mozambique Channel. Mayotte consists of a main island, Grande-Terre or Maore and a smaller island, Petite-Terre or Pamanzi. They are surrounded by several small islands. As far as we know there are no underground sites on the islands.
The islands are of volcanic origin, created by a hotspot and raising from the sea floor to about 660 m asl. Volcanic activity started about 7.7 Ma ago in the south, ceasing about 2.7 Ma ago. Due to the volcanic geology only volcanic caves are possible, lava tubes are close to the surface and tend to collapse after a few ten-thousand years. So there are no volcanic caves. The islands have a barrier reef, but as far as we know, no exposed layers of limestone which could contain karst caves. Also, there are no artificial underground structures which are open to the public.