Every city in the world has some secrets lying in its underground. Starting from the composition of the materials that make up the ground, to the marvels of history buried under it, every city has its bit of past which geologists excavate from time to time for their research.

According to Forbes, while setting up the tunnels for tube rail transportation, the geologists at many times uncover the everyday lives of the ones who went before. Geologists play a key role in planning and development of all major urban infrastructure projects along with the archaeologists. They always look for the presence of any buried articles with the use of ground-penetrating radar at the start of any excavation project. Another obvious aspect is the collection of samples from boreholes to find out the detailed composition of the underlying rocks and its water content, defining the ancient past of a city.

During the construction of the Mexico City Metro, researchers unearthed an Aztec pyramid, the carefully buried remains of 60 ancient human bodies, and a big collection of bones of the ancient elephants called mammoths. A collection of Byzantine shipwrecks were found lying at the Yenikapi Subway Station site in Istanbul. The long- lost port city of Theodosius saw the unearthing of almost 35000 bones, coins and ceramics. It also uncovered thousands of human footprints that went back to the Neolithic period.

In London, the Crossrail project has seen more than 100 archeologists since 2009 for its construction. During their course of action, they have found almost 10000 items across 40 sites, which includes the bones of prehistoric animals, ancient coins, and the remains of a Roman road. The discoveries reportedly span across 55 million years of London's history.

According to the Guardian, Malta has famous secret tunnels in Valletta, the capital city. It has earned the city the pride of being declared as a UNESCO world heritage site. It is a city of fortresses which has been used as set for blockbuster films like "Gladiator", "Troy", and the famous TV series "game of Thrones". The whole city is literally a hidden bunker. The unmatched underground sculpture has made the Maltese Government claim it as "Europe's first planned city".  

Along with the above-mentioned tunnels, a number of ancient pieces ranging from bison bones to ceramic wares were found during the excavation by geologists. The areas where these things were found lie at the Royal Oak Portal, Canary Wharf, Canning Town and other places.