Which underground feature is formed when a
stalactite joins to a stalagmite?

Pillars are formed when a stalagmite and a stalactite join together. When the water runs down the wall calcite is deposited in the same way as the stalactite and stalagmite to form a ridge which joins to form sheet of calcite called a curtain. Calcite formations can be seen at the Aillwee caves.

They can be called columns, pillars or stalagnates

Explanation:

answer called a epicycle;

ask an adult bc this is too hardcore