The Pongo ('gate' in Quechua) de Manseriche is 3 miles (4.8 km) long, located at 4° 27′ 30″ south latitude and 77° 34′ 51″ west longitude, just below the mouth of the Río Santiago, and between it and the old missionary station of Borja.

He fitted out an expedition at Loja in Ecuador, descended the Rio Santiago to the Marañón, passed through the Pongo in 1557 and invaded the country of the Maina Indians.

Later, the missionaries of Cajamarca and Cusco established many missions in the Maynas, and made extensive use of the Pongo de Manseriche as an avenue of communication with their several convents on the Andean plateau.

According to their accounts, the huge rent in the Andes, the Pongo, is about five or six miles (9.7 km) long, and in places not more than 80 feet (24 m) wide, and is a frightful series of torrents and whirlpools interspersed with rocks.

There is an ancient tradition of the indigenous people of the vicinity that one of their gods descended the Marañón and another ascended the Amazon to communicate with him.