I've always wondered that.
There are some products that claim to break it downThe source is an ad on Youtube for stuff that might do the jobNever tried it myself.
If you mean how are the supports for a bridge that are underwater are madeSome bridges will have the supports that are pre-cast concreteThese pieces are made on land, and then using a pile-driver are driven into the sea/lake/river bed until they reach a depth determined by an engineerOther bridges the construction company will build a coffer-dam around the areaThis is a temporary barrier that will keep out most of the waterThe water that does get in will be pumped outThe construction company then has a mostly dry area to build the support structuresFor a tunnelLike the chunnel between Britan and France, you get a large machine to core through the stoneYou then come in and pour concrete to make it structurally soundYou can also use the coffer-dam construction mentioned aboveYou section off the areaBuild the tunnelLet the water back in over the tunnelYou can also build the tunnel in sections with pre-castYou then trench out where the tunnel will go and drop it in the waterYou will need some underwater construction to seal the piece togetherPump out the water on the inside an finish construction.
I think acetone melts it but might be harsh on painted surface, might try goo gone or maybe WD40 to finnish with
