Trying to recall a couple book titles

Hi all, I hope someone here can help me; I’m trying to find the titles of two books that I read in elementary school, back in the 1970s.

One was about a group of people who somehow travel to the bottom of the ocean and find a city of amphibious “frog-people”. The cover showed this “frog-man” swimming up towards the reader. Unfortunately, that’s all I can remember. I don’t remember the storyline at all, or what happens to the people.

The other book was about a girl in New York City who sneaks into an abandoned house and steals a bracelet, and a ghost follows her and eventually takes the bracelet back. It may have been a short story in an anthology of some type, but I’m fairly sure it was a novel on its own.

Not much to go on, but do these books sound familiar to anyone? Any Boomers or Gen-Xers out there who remember such stories from their childhood?

Thanks for your help!

by InterestingAnt438