I'm not picking an argument, I have just seen a lot of temporarily clandestine deals go down in Belize.
It's a controversial enterprise, only makes sense to keep things to yourself.
Talk to the guy if he's still around who started a project 4 blocks south.
He hit a complete void 2 feet down, water only.
He had to put dozens of columns under foundation that bottomed out at 16 feet if I remember right.
The place was for sale for a while, he had the pictures.
That's just one "shallow" isolated chamber that can be verified, there may well be much deeper ones.
The map I saw of the cave had many branches that went sideways.
I happened to watch them drive some of the ones at the split.
They all hit the soft material you describe.
If there was a void they would have fallen right in.
They got lucky so far, but the fact remains this has to be verified.
This link says there are many large caves under the Caye, according to a long time native.
http://cayecaulker.org/history/