The Hypogaeum is a the last large layer of the Subterrranean Realm. It contains a thousand tunnels and chambers, huge seas of lava, massive underground lakes, steam tunnels, and several nations of warrens.

The Lava Seas:

1. Dalkan Sea: East of Utch-Shaul Lake, next to a steam tunnel, south of Yaamnach.
2. Fulkarn Sea: South of the Shadow Tunnels.
3. Hanicarn Sea: Northwestern sector of the world, north of Tull-Shaddach.
4. Lhaakheen Sea: In the southern sector of the world, southeast of Coesania.
5. Lhon Sea: West of Ennoshuam.
6. Naalgoru Sea: South of the stream tunnel connecting Dalkan Sea and two lakes.
7. Ottuk Sea: Next to Finjoek.
8. Pich Sea: Between Nozu-Wold and Tull-Shaddach.
9. Qadeen Qaisal Sea: South of Aurumvorasia, north of Coesania.
10. Redclaw Sea: North of Yaamnach, south of Hemduin Lake and a steam tunnel.
11. Sturgaal Sea: Northwest of Ennoshuam, east of the Shadow Tunnels.
12. Tiamat's Blood: East of the Shadow Tunnels, south of Vezeem.
13. Vishaz Sea: Southwest of Vezeem.
14. Xarlat Sea: North of Rezsthane.

The Lakes:

1. Golaaris Lake: Northeast of Finjoek.
2. Hemduin Lake: North of Yaamnach, connected to the steam tunnel connecting to Redclaw Sea.
3. Jorian Lake: North of Jorian.
4. Kudruch Lake: South of the Caldan Run, connected to the steam tunnel connecting Dalkan Sea, Naalgoru Sea, and Utch-Shaul Lake.
5. Marvant Lake: The largest underground lake; south of Isalant, surrounded by its own namesake nation. Mixed races.
6. Mysona Lake: Lake of the Lost Moon, between Rezsthane and Ennoshuam.
7. Orech Lake: Btewen Yaamnach and Stonedelve.
8. Pocket Lakes: Inaccessible, unknown lakes unconnected to any outside tunnels.
9. Pulgrom Lake: Southwest of Finjoek.
10. Undine Pool: Between the Caldan Run and Coesania.
11. Utch-Shaul Lake: East of Wuordon, connected to the steam tunnesl connecting Dalkan Sea, Naalgoru Sea, and Kudruch Lake.
12. Wise Lake: South of Jorian.
13. X'ibara Lake: West of Rezsthane, connected to the steam tunnel connecting to Lhaakheen Sea.
14. Zural Lake: East of Coesania, connected to the steam tunnel connecting to Sturgaal Sea.

Steam: There are five steam tunnels, each where a lake and a lava sea connect.

The Countries:

1. Aurumvorasia: The sub-Mati nation of living gold.
2. Braughaddrusk: Beneath Starfall, where the Braughaddruskr chthonians dwell.
3. Broken Tunnels: Shattered, broken series of tunnels under the Palhuric Ocean. Too dangerous for anything to enter safely.
4. The Caldan Run: A nation under the Wyren Ocean.
5. Coesania: Beneath Taggarus, this is where the coesanians dwell in their massive, queen-lead warren.
6. Ennoshuam: Beneath Dabusen, where the Ennosh chthonians etch the laws of the land onto the walls of their tunnels.
7. Finjoek: The gondal nation beneath Palhur.
8. Ghuraand Kazad: The great Akazadian chamber under Lyrilla.
9. Hrau'kharn: The country of the sard chthonians.
10. Jaugandaria: The cruel Jaugandarian chthonian nation rules here.
11. Jorian: Beneath the Black Ocean.
12. Khurrid: Beneath Wymmera. Where the Khurridan chthonians dwell.
13. Kitakaane: Beneath the Great Southern Ocean and Taggarus.
14. Living Mines: Beneath Starfall. Where living iron dwells, where dalim is found, and where kraman have their mines.
15. Marvant: The vast lakeside nation beneath Ranu and the Aurul Ocean.
16. Nhadrokhar: The country under Palhur where the Nhadrokharian chthonians build vast structures.
17. Nozu-Wold: The mushroom kingdom beneath Lyrilla.
18. Rezsthane: Beneath the Great Southern Ocean.
19. Roots of the World: Also pass through this layer.
20. The Shadow Tunnels: Where dwells eldritch horros beneath Ansulym, the Aurul and Black Oceans.
21. Stonedelve: Where dwell the Stonedelver elf peoples under Lyrilla.
22. Tull-Shaddach: Beneath the Altic Ocean, Palhuric Ocean, and Starfall.
23. Vezeem: Beneath Ansulym.
24. Wuordon: Beneath Palhur and the Palhuric Ocean.
25. Yaamnach: Beneath the Wyren Ocean and Starfall.
26. Zul'Mornak: The [[big-eared troll] tunnels beneath Jesenya.

Red is lava, blue is water, yellow is steam. The shaded tunnels are the Shadow Tunnels.