At some point you've been in an old building (or even a new one where the contractors tried to save a few bucks) and observed cupped floorboards. How does that happen?
European floor manufacturer Mafi explains it on their website: Because floor planks and strips physically expand and contract dramatically across their grain direction with corresponding increases and decreases in moisture content, they are liable to distortion and can generate enormous compressive forces. Wood floors have been known to expand to such an extent and with sufficient force, in extreme cases, as to crack or dislodge the perimeter walls. These distortion effects are most evident in non-engineered solid wood floors. The planks or strips can also expand and contract to cause both costly and unsightly effects including, debonding from the sub-floor, excessive gapping and a pronounced cupping effect.
If attempts are made to correct cupping (concave curving) by sanding the floor, then there is the real danger of ending up with planks suffering from crowning as the floor, due to a further change in temperature and humidity, regains its initial form.
Needless to say, Mafi's engineered flooring solutions don't suffer from this problem. The company uses the same wood on the bottom of their three-layered sandwich as on the top, ensuring they have identical expansion/contraction capabilities; these outer layers are bonded to a central core that resists movement.
Mafi has also come up with some beautiful milled flooring designs and devised a proprietary method of treating the wood, eschewing lacquer in favor of oil:
You can see more of their products here, and learn more about wood flooring here.