**Key Words**:

To aid in your search, you might want to try searching the term "[**Fossorial**](https://en.wikipedia.org/wiki/Fossorial)."

 - Fossorial animals are animals that are adapted to digging and life underground.

   - Though note: [Lessa et al (2008)](https://www.researchgate.net/profile/Matias_Mora/publication/229878464_Evolution_of_morphological_adaptations_for_digging_in_living_and_extinct_ctenomyid_and_octodontid_rodents_ADAPTATIONS_FOR_DIGGING_IN_CTENOMYIDS_AND_OCTODONTIDS/links/0046351b9fee74a47c000000.pdf) use **fossorial** to describe species that spend a substantial fraction of their lives outside their burrows, while they use ***subterranean*** to describe species that perform most activities underground.  

You might also want to examine a list of [troglobites](https://en.wikipedia.org/wiki/List_of_troglobites).
 
 - Troglobites are animals that live entirely in the dark parts of caves.

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**Big Species**

I think large specimens of @JayCkat's suggested species ([***Microchaetus rappi***](https://en.wikipedia.org/wiki/Microchaetus_rappi)) will be tough to "beat", with large specimens reaching 6.7 m and 1.5 kg. 

However, here's a list of other species anyways:


Some **extant large species of fossorial animals**:

 - [**Cape dune mole-rat**](https://en.wikipedia.org/wiki/Cape_dune_mole-rat) (*Bathyergus suillus*): 27-35 cm (up to 39cm including tail); 570-1350 g

   - Supposedly, some [blesmols](https://en.wikipedia.org/wiki/Blesmol) can reach a weight of 1800 g. 

 - [**Russian Desman**](https://en.wikipedia.org/wiki/Russian_desman) (*Desmana moschata*): 18-21 cm (up to 41 cm including the tail); 400-520 g

 - [**Giant Armadillo**](https://en.wikipedia.org/wiki/Giant_armadillo) (*Priodontes maximus*): up to 1.5 m; up to 50 kg

   - It spends all day in underground burrows though it hunts above ground at night. 

 - [**Olm**](https://en.wikipedia.org/wiki/Olm) (*Proteus anguinus*): up to 40 cm long. 


[![Cape dune mole-rat][1]][1]

<sup> [Cape dune mole-rat](http://www.ispotnature.org/node/837957) </sup>

[![Giant Armadillo][2]][2]

<sup> [Giant Armadillo](https://www.youtube.com/watch?v=zM2o4XYlBzM) </sup>


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The only info I could find via a quick search for **prehistoric fossorial animals**:

 - A series of 240 million year old underground tunnels/chambers suggest some burrowing species lived there. [Source: [Seeker](http://www.seeker.com/pre-dino-subterranean-world-discovered-1765411287.html)].

   > "You should imagine the tracemaker as a stout, short-bodied, four-legged animal with a short tail and short neck...The trunk was about 20-25 centimeters (8-10 inches) in length.

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**Bonus**: 
How about the ***deepest*** living animal?

 - That award goes to the ["**Devil Worm**"](http://news.nationalgeographic.com/news/2011/06/110601-deepest-worm-earth-devil-science-animals-life/) (*Halicephalobus mephisto*), a nematode that has been found living at 3.6 km below the surface!! ...(tied w/ *Plectus aquatilis*).

 - Though at 0.5 mm, it's definitely not winning the *largest* trophy :p.

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<sup> *Citations*: </sup>

<sup>- Lessa, E. P., Vassallo, A. I., Verzi, D. H., & Mora, M. S. (2008). Evolution of morphological adaptations for digging in living and extinct ctenomyid and octodontid rodents. Biological Journal of the Linnean Society, 95(2), 267-283. </sup>


  [1]: https://i.sstatic.net/Tch1u.jpg
  [2]: https://i.sstatic.net/3OPSH.jpg