"You have chosen wisely. But the Grail cannot pass beyond the great seal. That is the boundary, and the price of immortality."―Grail Knight to Indiana Jones[src]
The Great Seal was a marking point inside the Temple of the Sun.
When Indiana Jones recovered the Holy Grail from the temple in 1938, the artifact's guardian warned that the Grail could not be taken beyond the Great Seal. However, Jones' companion, Elsa Schneider, defied this warning which triggered the temple's collapse and ultimately cost Schneider her life.