Horses and Player take damage and die when trying to ride underground

I have a large underground pit mine. 3 horses spawned long before the mine got deep. When I tried to ride them out of the mine from deep underground, 2 of the 3 horses took damage, 1 died and my player died with it. I believe it has to do with being in a place that once was deep underground. Please look into this and resolve! I was level 32 before this and lost almost all my Exp!