Balsa

Balsa is a hardwood[?] - meaning that the tree is deciduous, broadleafed and slow growing, even though balsa trees grow to 90 feet high in seven years or so. The timber itself is very soft and light with a coarse open grain. It is much used for model making and was famously used by Thor Heyerdahl in his raft Kon-Tiki - which has its own museum http://www.museumsnett.no/kon-tiki/.

The density of balsa wood ranges from 3 to 30 pounds per cubic foot, with a typical density of about 10 pounds (about one third the density of ordinary wood).