In a booming sector where the biggest ships have doubled in size since 2000, pressure is growing to make cruising a greener, more sustainable way to travel

• Read more in this series

Toxic, filthy and cheap, the sludge-like substance known as heavy fuel oil has powered the shipping industry since the 1960s. What is perhaps less well known is that this same substance is still used to power more than half of cruise ships today, making what many choose as an alternative to flying one of the most environmentally damaging ways to travel.

The good news is that the industry, under pressure from environmentalists and new regulations, is adopting new technologies, energy saving designs and studying alternative fuels.