Sent in by Mike Waters at The Unofficial World of Nintendo!!!
In level 3-2 there is a boulder over some quicksand. Shoot the boulder until
it is destroyed. Then fall through the quicksand. Instead of dying, you will
end up in a secret area with some good bonuses. You'll still have to fight
and jump plenty (over holes this time instead of cacti), but you had to do
that above ground as well. Down here, however, you will be rewarded with
2 big hearts if you find them in their boxes. When you find these, Fievel
becomes capable of taking 5 hits instead of the usual 3.