Middle/YA Fantasy 3 Children Battle below ground

I read this book in the early 1980s. It is about a boy and a girl who visit their uncle for the summer and meet another boy. While traipsing around they find a door that leads to an underground world. There is a long flight of stone steps and a boat. And creatures that are fighting a battle of good against evil. It is definitely not Over Sea and Under Stone.

Any assistance to identify that book is most appreciated.

Starting point in case there is a remote chance it is here.
Underground/dome to outside - children's/young adult science fiction
https://www.librarything.com/list/588/all/Underground%25252Fdome-to-outside-chil...

Or, Maurice Gee, Under the mountain.

Weirdstone of Brisingamen?

The Silver Chair last half is underground, 3 main characters, good vs evil. No uncle or door is this book of the series, though.

Not Under the Mountain. Not Weirdstone of Brisingamen. Not anything by C.S. Lewis.

bookel, that is a great list; thanks. The book I am looking for isn't there.
It also isn't Maurice Gee, Under the mountain., Weirdstone of Brisingamen, and it isn't anything in the Narnia chronicles.

Lawrence Leonard, The horn of mortal danger?
I can never recall that title and author, but it is memorable ... except I'm not sure if details fit. It has been a few years since I read it.

Maybe "Beneath the Hill" by Jane Louise Curry, but it has been years since I read the book.

It isn't Jane Louise Curry,: Beneath the Hill. There is no treasure hunt and the adult relative is definitely male.

I have never read Lawrence Leonard, The horn of Mortal Danger, but based on the blurb, that isn't it. No railroad tunnels in this one. Just underground-- no railroad or other manmade tunnels. It sounds interesting though, I will add it as a "too read".

bump

It was also not The Dark Portal by Robin Jarvis because the protagonists were human.

The Secret World of Og by Pierre Berton has five children, no uncle. They do go underground, and there is a boat.

Thanks MissSquish. Sounds delightful but that's not it. Too many children. The book I am thinking of is only 3 children and only two of them are related-- the third child is a neighbor.

Do you have an idea of where/ what country it was set? And when it might have been written or what era it was set in?

humouress I read it in 1980s and it was a contemporary book. I don't remember where it was set-- maybe US or maybe England.

Hmm. You’ve already ruled out the Narnia books. The only one I can think of is The City of Ember but that starts off underground.

Can rule out Chalk, The Gnomids I think.

How about “The Secret Lake” by Karen Ingliss or *maybe* “Magic by the Lake” by Edward Eager?

Inglis: The secret lake
Eager: Magic by the lake
Links (just use square brackets around it) and author surname spelling helped.

Whispering Mountain by Joan Aiken?

If you make the right choices while reading By Balloon to the Sahara, it leads the three children through a door to a boat on an underground river.