Each blast creates a shockwave that stuns Bloons at impact point and pops Bloons further out.
~ BTD6 description
Shell Shock is the third upgrade of Path 1 for the Mortar Monkey in BTD6. It allows the mortar to create secondary explosions that can stun affected bloons for 0.5 seconds that were damaged near the center of the main explosion radius and/or pop one or two layers of bloon depending on how close to the center of original explosion the bloons were. The secondary explosions will pop up to 45 bloons, just like previous upgrades.
It costs $925 on Easy, $1100 on Medium, $1190 on Hard, and $1320 on Impoppable.
The name is a reference to actual shell shock, though that applies to people induced with trauma rather than referring to stunning bloons.
Currently, when tested among 999 Red Bloons of 0 spacing, the damage count would inconsistently add +90 to the damage count even though pop limit is usually 45. This is because occasionally the secondary explosion may not actually hit those Red Bloons. This also occurs for subsequent upgrades.
Shell Shock was originally going to be named "Shockwave". This was evidenced while some BTD6 players were datamining the in-game code of BTD6.