The undertows (spelt correctly in my review) are bull. Invisible obstacles that choose to screw you over at the worst time for you, but never help you in any way, so you have to send one set of subs at a time because the undertows will, repeat, will choose to make the subs fly in every which direction when you can't stop them. I got literal seconds from winning, only for the sub to, after I stopped telling it to halt, get flung halfway across the screen for no reason at all.