The London Mithraeum is a very old building in London, England that was built a very long time ago when the Roman Empire ruled there. It was a special place where people would go to worship Mithras, who was a god that the Romans believed in. Mithras was a really important god for the people who worshiped him because they believed he would help them in life and protect them from bad things.
The London Mithraeum was actually built underground and people would have to climb down a staircase to get inside. Once inside, there was a really big room that was used for worshiping Mithras. In this room, there were also lots of really cool things to see like statues of different gods, fancy decorations on the walls, and even a big altar where people would offer gifts to Mithras.
Today, people can visit the London Mithraeum and see all of these amazing things that were built so long ago. Even though it's underground, there are big lights so everyone can see everything really clearly. The London Mithraeum is a really special place that helps us better understand what life was like for people during the time of the Roman Empire.