Shadrach says, "You make a good point about the possibility of a second bomb, but I have to think any other explosives would've gone off when the first one exploded. I just want to examine the explosion and I fear this is less risky than taking the long way. You all can share this information you have about Anna being in the Undermountain once we make sure we are secure here."

The pit reopens and a few thin tendrils of smoke emerge. There are several grappling hooks on the edge of the pit that the kobolds used to make their attack, he checks a couple of them for a suitable one, and begins his descent.

Aria does the same... noticing which ones he didn't choose and finds one herself.... "No one should ever go down there alone..."   She too begins her descent

While they go into the Undermountain, what do the rest of you want to do? Anything? Or just use this opportunity to take a short rest?

Quote from Evlintentions >>

Aria does the same... noticing which ones he didn't choose and finds one herself.... "No one should ever go down there alone..."   She too begins her descent

That line hits Usul just right. He's been alone for too long... He looks down into his pocket to Dib and whispers some words to him*, takes a deep breath, says "**** it...", and finds a rope of his own. Though it takes him a good 3 rounds to do this, and he doesn't try to speed down to catch up. He'll also try to be as quiet as possible. stealth 19

Crush would have started climbing down the rope the moment the portal was opened back up. He wasn't trying to be sneaky or waiting around for permission.

Usul is able to quietly go down into the Undermountain. Crush, Shadrach, and Aria are not trying to be sneaky. Is Usul trying to hide from them as well?

Sen would like to fret.

A young boy in the company of his father notices Sen's fretting. Leaving his father behind, but not out of sight, he says to Sen, "Are you OK? Do you think there will be more of them? I saw the magic you used, that was so great!" 

Quote from SteveThaiBinh >>

Sen would like to fret.

 (( This a bonus action? lol :) ))

For Sen it's a free action. 

(More like his superpower)

Sen regards the child for a moment.  I don't know.  I don't think so.   Do you get a lot of kobolds in Waterdeep?   

Distracted, he turns back to staring at where Aria and the others started their descent into the Undermountain.  

The boy says, "I don't think so. I never saw a raid before. Sometimes my brothers go hunting down there and I got to see a dead kobold once. This is way cooler though. Where did you learn to do your magic? I never met a wizard before. Can you do other magic?"

Once at the bottom of the portal, Crush would draw his bloody hand axes and look around for any signs of wounded kobolds. The orc would quickly look around the area and take anything of value from any kobold corpses, kicking over their lifeless bodies and taking anything useful from them. 

Oh really?  asks Sen. Your brothers must be very brave.  Are they here?  I would like to meet them.  As they talk, Sen uses Minor Illusion to produce a series of images that will interest the boy; (depending on his age, statues of animals if he's younger, weapons if he's older.)  I mostly do illusions.  I'm not very good yet.

The boy is thoroughly amused by Sen's images, especially at one of a puppy. He says, "Oh no, they're not here now. They were supposed to meet us here before they went hunting in the Undermountain. They were out late last night. Dad says they were being ir.. irry...irreypondsbubbles"

Those in the pit see carnage - kobold parts are everywhere. The bomb was housed in a barrel, the bottom third of which is still in tact. The metal bands that had been higher up on the barrel were ripped apart and are among the debris. 

Crush wouldn't find any kobolds still alive. There are some coins scattered through the wreckage to the tune of 14 gold (this is without making any kind of rolls, FYI).

Shadrach kneels down and examines the remains of the barrel. "This doesn't appear to be simple alchemist's fire. Surely they couldn't have come up with something like this themselves... Did they find this by accident? No, that couldn't be... Who could have given kobolds something like this? How strange." He begins to search more thoroughly through the wreckage. 

Since it seems mostly safe, Usul will come out of hiding and start to search as well, looking for anything interesting, though of course also shiny coin shaped things ^^, saying "What are you looking for?" investigation 17

Aria will try and take a look around the area... see what she can find or if anything stands out.

Underneath a pile of kobold pieces -- it's unclear if this was one kobold or two -- Usul finds a pouch containing 152 gold. 
Aria doesn't see anything in particular.

(I would encourage other investigation checks)

Shadrach stands up and looks at the stone columns and wooden buttresses that support the portal and act as part of the foundation of the building above. They appear scorched, and there's some damage, but not anything that can't be fixed. There's no immediate danger of the building collapsing. "A miracle, praise Tymora. The building isn't going to collapse on us." To Usul, he says, "Doesn't this seem odd to you? To any of you? Kobolds aren't smart enough to build a bomb and bring it with them on a raid like this. They're barely intelligent enough to organize a raid like this. They certainly aren't brave enough. I'm going to have to look into this -- but --" he turns to Crush (who I imagine is smashing dead kobolds into pulp), "You said Anna is down here, somewhere?"

EDIT: Rounding the gold up to an even 160 in the pouch. Usul would immediately assume that this was the kobold's payment for attacking the Yawning Portal. 

Usul quickly pours a handful of coins (48) from the pouch, hoping to keep that hidden from anyone watching (slight of hand: 20), and then holds the pouch out saying "It seems that they got paid... and well... There is no reason kobolds would have this much coin on them."

If anyone wants to roll perception, DC 24, to see if your character notices Usul hiding the pouch, go for it.