What causes Sinkholes?

“Sinkholes typically develop slowly as bedrock is whittled away by water turned acidic from absorbing carbon dioxide and interacting with plants. Rainwater obviously plays a role, but unseen water also matters. As the acidic water dissolves rock, it carves out conduits, or underground passages, for water. These conduits in turn help to develop underground basins known as recharge areas. Recharge areas contribute to the formation of sinkholes as water flowing to and from them and into the subsurface (the earth overhead) erodes bedrock. When water floods a developing sinkhole, some of the topsoil and other material can be caught in the conduits, further trapping water and limiting its ability to flow outward.”

HowStuffWorks

@syz Thanks, that is quite interesting.

Also, overpopulation consuming the water supply.

Places where sinkholes form are called “Karst landscapes” Or just Karst.
Chemistry of karst landscapes

Karst lake (Doberdo’ del Lago, Italy), from underground water springing into a depression. This lake has no surface inlet or outlet.Karst landforms are generally the result of mildly acidic water acting on soluble bedrock such as limestone or dolostone. The carbonic acid that causes these features is formed as rain passes through the atmosphere picking up CO2, which dissolves in the water. Once the rain reaches the ground, it may pass through soil that may provide further CO2 to form a weak carbonic acid solution: H2O + CO2 → H2CO3. Recent studies of sulfates in karst waters suggests sulfuric and hydrosulfuric acids may also play an important role in karst formation.

This mildly acidic water begins to dissolve the surface and any fractures or bedding planes in the limestone bedrock. Over time these fractures enlarge as the bedrock continues to dissolve. Openings in the rock increase in size, and an underground drainage system begins to develop, allowing more water to pass through and accelerating the formation of underground karst features.

Somewhat less common than this limestone karst is gypsum karst, where the solubility of the mineral gypsum provides many similar structures to the dissolution and redeposition of calcium carbonate.

http://www.newworldencyclopedia.org/entry/Karst_topography