Could it be At the Earth's Core by Edgar Rice Burroughs?

I have not read it, but in Wikipedia's description, a miner named David Innes has an experimental excavating vehicle called an "iron mole".

There is a problem with the vehicle and they end up burrowing 500 miles into the Earth's crust and discover a land underneath -- a kind of hollow Earth -- called Pellucidar.

It has several different kinds of creatures, including the Mahars, which are intelligent flying reptiles.

Does this ring a bell?

You can read the Wikipedia entry on it here

Could it be At the Earth's Core (1914) by Edgar Rice Burroughs?

I have not read it, but in Wikipedia's description, a miner named David Innes has an experimental excavating vehicle called an "iron mole".

There is a problem with the vehicle and they end up burrowing 500 miles into the Earth's crust and discover a land underneath -- a kind of hollow Earth -- called Pellucidar.

It has several different kinds of creatures, including the Mahars, which are intelligent flying reptiles.