West of the Panama Canal's Caribbean end, the land goes quiet surprisingly fast. You cross from the Colón side on the Atlantic Bridge, the long cable-stayed span that finally replaced the old swing bridge and the ferry over the canal entrance, and within minutes the container cranes give way to rainforest, old American base buildings and a narrow road heading north toward the mouth of the Chagres River.
History is thick here. The Chagres was the original highway across the isthmus: Spanish treasure came down from Panama on mules and river boats and was shipped out from this coast, which is why the Crown built Fort San Lorenzo on the cliff above the river mouth. Henry Morgan's men stormed it in 1671 on their way to sack old Panama, and the ruins, together with Portobelo further east, are on the UNESCO World Heritage list. Then came the gold rush railway, the failed French canal, and finally the American canal, whose Gatun Dam drowned the Chagres valley to create Gatun Lake. The former Fort Sherman area you ride through was a US jungle training base until the canal was handed over.
The road to San Lorenzo is the reason to come. It is mostly paved but narrow, patched and occasionally broken, with standing water after the afternoon storms and the odd fallen branch. The forest of the San Lorenzo protected area closes in on both sides; howler monkeys are more often heard than seen, and toucans, coatis and sloths are real possibilities if you ride slowly. Ride in the morning. From May to December the rain arrives most afternoons, and the heat and humidity are heavy all year, so carry water and give yourself time for the walk around the fort.
Practical matters are simple: there is nothing out on the San Lorenzo road, so top up in Colón or at the stations around Sabanitas on the Transístmica before you cross. Colón is a gritty port city with a famous free zone and a rough reputation in parts, so most riders treat it as a place to fill the tank and buy food rather than linger. If a ship is passing through Agua Clara or Gatun while you wait at the bridge or the lock crossing, watch it. Seeing a vessel the size of an apartment block rise through the locks in the middle of the jungle never gets old.