The city's most staggering discovery is the brutalist National Library building, a structure so bizarre and seemingly out of place that many are left unsure as to what to make of it. It still is a perfectly functional library, so do drop in to get a good look at what's inside – you might just chance upon an English-language exhibition.

A feat of Spanish colonial military architecture, the Castillo San Felipe de Barajas is one of Cartagena's most prized attractions. The fortress, whose construction finished in the late 18th century, was never seized — and not for a lack of attempted forays. Part of its sophisticated system of underground tunnels is open to the public. A guided tour (audio guides available) is highly recommended.

Tens of millions of red land crabs live on Christmas Island. They are the Island's keystone species, because they play a vital role recycling nutrients and shaping and maintaining the structure of the rainforest. At the beginning of the wet season (usually in October/November), most adult Red Crabs suddenly begin their spectacular migration from the forest to the coast, to breed and release eggs into the sea. Breeding is usually synchronised island wide. The rains provide moist overcast conditions for crabs to make their long and difficult journey to the sea. The timing of the migration breeding sequence is also linked to the phases of the moon, so that eggs may be released by the female Red Crabs into the sea precisely at the turn of the high tide during the last quarter of the moon. It is thought that this occurs at this time because there is the least difference between high and low tides. The sea level at the base of the cliffs and on the beaches, where the females release their eggs, at this time varies the least for a longer period, and it is therefore safer for the females approaching the water's edge to release their eggs. Sometimes there are earlier and later migrations of smaller numbers of crabs, but all migrations retain this same lunar rhythm. The main migration commences on the plateau and can last up to 18 days. Masses of crabs gather into broad "streams" as they move toward the coast, climbing down high inland cliff faces, and over or around all obstacles in their way, following routes used year after year for both downward and return migrations. Movement peaks in the early morning and late afternoons when it is cooler and there is more shade. If caught in open areas, in unshaded heat, the crabs soon lose vital body moisture and die. The crabs will migrate to the coast, where the males will dig mating burrows and they will mate. After mating, the males will commence their return migration. The females will brood their eggs for 12-13 days before emerging from the burrows to commence spawning. The females will commence their return migration immediately after spawning. The eggs hatch into free swimming larvae immediately after they are dropped into the sea. The larvae grow through several stages in the ocean for over four weeks before emerging from the sea to become tiny crabs.