Kaldur heads back towards the hangar, which contains the underwater entrance to the mountain. He won't leave the cave—that wouldn't be fair—but he can sit underwater next to the bay doors which lead out into the Pacific Ocean. If he picks the right spot, he won't be visible from the surface. 

Batman glances up from his work updating the hangar's security systems. He notes the speed at which Aqualad is moving.

"Trouble?"

Kaldur slows from the military jog he had unconsciously fallen into.

"Ah—no. I am merely participating in...an important team-building exercise," he explains, straight-faced and with only a little hesitation. "My apologies for disturbing your work."

He heads over to the ramp that leads down to the water, hoping Rain wasn't paying enough attention to overhear that conversation. 

Ah. That kind of training exercise. Sounds like an excellent idea, really. They'll introduce themselves to each other and familiarize themselves with their teammates' tactics, skills, and personality.

And Robin is definitely going to win.

"No apology necessary. Carry on."

Rain started paying attention to that conversation when he heard Kaldur's voice. Yep, there's the splash, called it.

Are there any other clues Rain can pick up from listening while everyone hides?

Nate isn't exactly the quietest when he's running this fast, especially since he had to dodge a few Leaguers himself while he was racing through the corridors. 

Judging by the volume and the echoes, he's somewhere on the far side of the mountain by the time he slows down. 

M'kel is barely making a sound. Floating off the ground means there are no footsteps to give her away, and if she's invisible then she won't give away her position by needing to talk to an adult like Kaldur just did. 

When Rain turns his attention to her, she holds her breath and stretches her control of her physiology to slow down and muffle her heartbeat, just for a moment or two until he focuses on someone else. 

Robin has found a hiding place and is currently holding very still.

He's practising a breathing exercise Batman taught him to slow down his heartbeat. Unlike M'kel, he'll be able to sustain this level of quiet for a while. 

Yvette's footsteps are ordinary, and she has to awkwardly stammer through some explanations to confused members of the Justice League on her way to her hiding place—

—And then they disappear. Her footsteps, her breathing, her heartbeat. Gone. Probably by magic.

"One hundred," says Rain, very quietly. He's smiling.

Kaldur first, then. He's closer and he knows (in a general sense) where he is. To the hangar!

Kaldur hasn't moved by the time Rain gets there. He's not immediately visible, having found a corner to hide behind, but his heartbeat is probably audible even through the water.

The problem will be reaching him, as he's picked one of the deepest points he could find without leaving the cavern. 

Mhm. Mhm. Yes, he sees.

If Kaldur isn't visible to Rain, then Rain isn't visible to Kaldur. So he can take a minute, remove his shoes, take off his shirt...

... and then, once all that's done, he looks at Batman. He smiles brightly.

"You have an underwater breathing thingy, right? Can I borrow it?"

"... I'll be expecting it back," says Batman, but he hands over the 'underwater breathing thingy.' "Undamaged."

"Yep!" agrees Rain, brightly.

He judges where precisely he needs to aim towards to land right on Kaldur. Yeah, he can make that jump. This decided, he takes a few deep breaths, expels all of the air from his lungs, puts in the underwater breathing thingy, and leaps. His aim is very good.

No oxygen in his lungs means no buoyancy, which means that even without an advantage in water, he is sinking like a stone. Right towards Kaldur.

Kaldur spent two years in the Shayeris military and has the reflexes to prove it.

His instinctive reaction to something dropping on him from above, even in a context where he isn't expecting an attack, is to hit it. Fortunately for Rain, he doesn't have his Water-Bearers to hand at the moment, so he's only punching with bare fists. Less fortunately, since Rain decided to dive feet-first, his blind swing connects with...a rather sensitive area. 

Ow, that's pain, that's sure some pain, ow ow ow ow ow—

—he has the presence of mind to lightly tap Kaldur before he shuts down entirely, and then ow he really doesn't have the presence of mind to make it back to the surface, ow ow ow ow ow.

He's sure glad he got that breathing thingy from Batman, because at least he is allowed to curl up and be pain itself for a while without having to worry about not being able to breathe underwater. Mhm. Yep. That was a good plan. Not that he can really tell right now. In case it wasn't clear: ow.

Kaldur recognises Rain after less than half a second and relaxes out of full alert.

It only takes him another half-second to remember that Rain can't breathe underwater on his own, which sends him right back into panic mode again until he notices the rebreather. Even with that, though, he should probably help his friend get to the surface. 

Rather than outright grabbing the person he just punched, he taps Rain lightly on the arm to get his attention. 

Rain's a little busy right now being agony itself, but - yeah, fine, hi Kaldur. He knows you're there. Good job. He makes a little thumbs up that could either mean 'I'm fine' or 'please get me to the surface.' It's not actually very clear.

(Elsewhere, M'kel finds Rain's commentary on the situation utterly hilarious.

But, ow, that was painful. Now she gets why Earth television always made such a big deal out of men getting hit there.) 

Kaldur decides to call that good enough; if he gets punched trying to help at this point, it'll only be fair.

He takes Rain's arm and starts towing him gently in the right direction. Not straight up—even with the water helping he doesn't want to try lifting a clone of Superman one-handed—but drifting upwards and in the direction of the ramp. 

Rain cooperates with this ferrying. Things slowly stop being entirely agony. By the time they reach the surface, he can think real actual person thoughts again.

He removes the rebreather, then coughs, "Good reflexes."

"My apologies," Kaldur says, holding out a hand to help him up. 

"I did not see you, and I simply reacted on instinct." 

"S'fine," he says, sincerely. He waves the rebreather at Kaldur in lieu of accepting the hand and getting up, though. "Return this to Batman for me please?"

Batman, for his part, is still updating the hangar's security systems. He looks to all the world like everything happening in this hangar is of utmost importance and is deathly serious about all of it.

Kaldur waits patiently for him to reach a stopping point, rather than interrupting. 

He doesn't know enough about technology to understand what Batman's doing, but it's interesting to watch all the same. Plus, this gives Rain a chance to get his breath back without making it obvious that Kaldur is waiting for him.