Scores of zombies spawning underground in tight compacted spaces is causing Minecraft Realms to severely lag then crash.
Zombies seem to build up over time depending on how long the user stays in the nearby chunks. Over sixty in one cave have been found before. Upon using spectator and creative modes to light the underwater lakes, lag and crashes began to dissipate.

Both underwater lakes and unexplored caves have been found with greater than fifty zombies cramped into them. Several of these "hellcaves" have been found at once, causing unprecedented lag.

To reproduce: Be on a realms server, stand in one area above underground lakes/caves, crash. Optionally use spectator to see zombies causing crash.