Just to get the big picture stuff out of the way, the biggest “that looked expansive” man-made disaster in human history was the Chornobyl disaster in 1986. While it can be hard to work out exact figures, as there are a number of disagreements on how to measure externalities and Soviet bookkeeping was often fanciful, some estimates suggest damages around $700 billion. 

It’s also a disaster of such magnitude that there are consistent, ongoing costs to keeping the damage in check, from maintaining the exclusion zone to maintenance on the protective constructions near the reactor itself. Fortunately, this sort of disaster is unlikely to repeat itself in this day and age. 

The next on the list, dwarfed by a number of earthquakes and hurricanes, is the 2010 Deepwater Horizon oil spill. It is still considered to be the largest marine oil spill in the history of the industry, dumping 780,000 cubic meters of oil into the Gulf of Mexico. Like most ecological disasters, it can be hard to measure the damage in the short term, but some estimates put it at $60 to $100 billion dollars.