Most Western translators translated ullam to heart
Ullam can be translated to inner most recess or inner most or Atman or heart. Since some of the translation was done for Western Civilization they used heart.
In reality, inner most recess is the correct way of saying it.
In that case it is the same as Lahiri Mahashaya said it as Kutastha
Brahmananda and Akho called it Brahmar Gufa
In Chandogya upanishad it is called Dahar Vidya.
But ask your guru where it is and how to get there
Bhavin