Subsidence

It is then, as his eyes travel to follow Michelangelo, that Leo spots the way the water is dripping thicker now, seeping through the concrete of the ceiling. There are signs he has carelessly neglected – the cracking on the stone, the groaning of pipes around them. His cheerful mood snaps.
Something must reflect this in his expression; Mikey pauses, a few feet ahead of him, face twisting to concern.
“Leo –“ he begins, but is interrupted when everything shudders. Rubble drizzles, then throws itself upon them as the roof collapses under the strain of the water pipes.
For a moment, Leonardo’s eyes meet his youngest brother’s and see the terror flash inside them. He barely has time to take another breath before the world caves in, and everything goes black.
~
While the brothers are out exploring a new sewer system, the whole thing collapses. With no way to get out and separated from each other, they all face their own struggles, injuries and fears as they fight to survive and find each other.
TLDR: a sewer caves in. Leonardo and his brothers are trapped because I like angst.