Exporting and Importing Issues

Hello, I wanted to make a highway that is more like a low viaduct, so I created a Terrain on Unity and brushed in the reliafs, but to do it I would have to import the unity's Terrain mesh into maya, so I could model the suspense highway following the changes in the reliefs of the terrain, but how can I export it from the .asset format to one more sympathetic for Autodesk Maya, How can I do that ?

Thanks to all!

 

https://www.assetstore.unity3d.com/en/#!/content/22250
https://www.assetstore.unity3d.com/en/#!/content/37184
https://www.assetstore.unity3d.com/en/#!/content/17079