You would think that after 20+ years since Doom 2 was released that every secret area has been discovered. Evidently there was one left undiscovered.
John Romero pasted a YouTube video showing how a Pain Elemental revealed the last official secret in the game.
Secret explained: A secret teleporter is marked as a discoverable “secret”, but when you touch it, you never enter the sector. So you would never get inside the teleporter sector to trigger the secret. The only way to trigger this secret is to be pushed by an enemy into it.
— 𝕵𝖔𝖍𝖓 𝕽𝖔𝖒𝖊𝖗𝖔 (@romero) August 31, 2018