We're told the team scrambled to add much-needed precautions to the massive ensemble, including handles that give them something to hold on to while inside.

The massive getup includes 3 towering heads ... each able to blow smoke. FYI, a costume like this probably isn't helmed by just one celeb -- we're thinking the 3 heads might be a clue.

The outfit is so over-the-top, it requires 5 times the number of staffers it normally takes to help the mystery singer(s) ready for show time.

While contestants usually put on their costumes in dressing rooms -- allowing them to keep their identity a mystery -- the Hydra has to do things differently.

We're told the costume's so big, the performers have to walk to the stage exposed, and get dressed there. That's means the crew has to lock down the set and build a barrier to wall off the stage, keeping everything hush hush.