Cholula, Mexico

This sprawling archaeological site is not for the faint of heart. With over 8 km of tunnels snaking through a massive 300 x 315 meter base, the Great Pyramid of Cholula is a monumental feat of engineering - but also a logistical headache. Visitors must navigate a web of fragmented excavations and restricted access, all while the local authorities struggle to balance preservation with development. And don't expect much hand-holding - there are few signs to explain the structures within. Bureaucracy is a persistent challenge, as 90 hectares of the 154-hectare site remains privately owned. Still, the sheer scale and history of this 25-meter tall, 4.45 million cubic meter pyramid make it a must-see for adventurous travelers with a passion for pre-Columbian archaeology.