Apperently some examples of floodings exist as well (in case someone comes along and claim floodings is an 'unlore' element hehe, such happends):
Soon after the winter broke. ... There were great floods after the snows, and the vale of Entwash became a vast fen.
- Appendix A
2912 Great floods devastate Enedwaith and Minhiriath.
- Appendix B
However I must concur with Torīn that it's strange for only Dwarves and Orcs, since those are the best diggers and miners;
`But in metalwork we cannot rival our fathers, many of whose. secrets are lost. ... Only in mining and building have we surpassed the old days. ... And the halls and cavernous streets under the earth with arches carved like trees; and the terraces and towers upon the Mountain's sides! Then you would see that we have not been idle.'
- Gloin, FotR; Many Meetings
Now goblins are cruel, wicked, and bad-hearted. They make no beautiful things, but they make many clever ones. They can tunnel and mine as well as any but the most skilled dwarves, when they take the trouble...
- The Hobbit