Teleport to a location without y coordinate in Minecraft?

So how does that work? I want to go to a mine I was looking for but I don't have a y coordinate.

You can with

/ tp 1000 ~ 1000

Teleport you to the X / Z coordinates 1000 and the Y axis remains as it is now.

Then you are at least in the desired position.

Now with

/ gamemode Spectator

go into Spectator Mode and fly down until the desired cave is found.
Then in a safe place again

/ gamemode survival

and done.

Just type ~ or any number

The y coordinate is the height