Ruins () are the remains of a civilization's architecture. The term refers to formerly intact structures that have fallen into a state of partial or total disrepair over time due to a variety of factors, such as lack of maintenance, deliberate destruction by humans, or uncontrollable destruction by natural phenomena. The most common root causes that yield ruins in their wake are natural disasters, armed conflict, and population decline, with many structures becoming progressively derelict over time due to long-term weathering and scavenging. There are famous ruins all over the world, with notable sites originating from ancient China, the Indus Valley and other regions of ancient India, ancient Iran, ancient Israel and Judea, ancient Iraq, ancient Greece, ancient Egypt, Roman sites throughout the Mediterranean Basin, and Incan and Mayan sites in the Americas. Ruins are of great importance to historians, archaeologists and anthropologists, whether they were once individual fortifications, places of worship, ancient universities, houses and utility buildings, or entire villages, towns, and cities. Many ruins have become UNESCO World Heritage Sites in recent years, to identify and preserve them as areas of outstanding value to humanity. Cities Ancient cities were often highly militarized and had fortified defensive settlements. In times of war, they were the central focus of armed conflict and would be sacked and ruined in defeat. Delhi, the capital of India, has been destroyed and ransacked seven to ten times and subsequently rebuilt. Every ruler decided to build the city in their own way either overlapping the ruins or next to the ruins. Ruins of seven cities of Delhi can still be traced in the modern-day city. Although less central to modern conflict, vast areas of 20th-century cities such as Warsaw, Dresden, Coventry, Stalingrad, Königsberg, and Berlin were left in ruins following World War II, and a number of major cities around the world – such as Beirut, Kabul, Sarajevo, Grozny, and Baghdad – have been partially or completely ruined in recent years as a result of more localized warfare. Entire cities have also been ruined, and some occasionally lost completely, to natural disasters. The ancient Roman city of Pompeii in modern-day Italy was completely destroyed during a volcanic eruption in the 1st century CE, and its uncovered ruins are now preserved as a World Heritage Site. The city of Lisbon in Portugal was also completely destroyed in 1755 by a massive earthquake and tsunami; and the