XPath difficulties when working against XML containing default namespace

Question

XPath is kicking my trash. I'm not sure why it's so hard for me to get it working. I paste below what I'm trying, and some quotes from the documentation.

The sanitized file:

<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2" xmlns:gx="http://www.google.com/kml/ext/2.2" xmlns:kml="http://www.opengis.net/kml/2.2" xmlns:atom="http://www.w3.org/2005/Atom" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<Document>
        <Style id="PolyStyle70412"></Style>
        <Folder>
                <Placemark id="default_starting_location"></Placemark>
                <Folder>
                        <name>LandParcels</name>
                        <visibility>0</visibility>
                        <Folder>
                                <name>LandParcels batch 1-2</name>
                                <visibility>0</visibility>
                                <Document id="LandParcels" xsi:schemaLocation="http://www.opengis.net/kml/2.2 http://schemas.opengis.net/kml/2.2.0/ogckml22.xsd http://www.google.com/kml/ext/2.2 http://code.google.com/apis/kml/schema/kml22gx.xsd">
                                        <name>LandParcels</name>
                                        <visibility>0</visibility>
                                        <Snippet maxLines="0"></Snippet>
                                        <description>text</description>
                                        <Style id="PolyStyle70"></Style>
                                        <Folder id="FeatureLayer7">
                                                <Placemark id="ID_70000">
                                                        <name>12345678</name>
                                                        <MultiGeometry>
                                                                <coordinates>100,200</coordinates>
                                                        </MultiGeometry>
                                                </Placemark>
                                        </Folder>
                                </Document>
                        </Folder>
                </Folder>
        </Folder>
</Document>
</kml>

The problem and expected result:

>>> from lxml import etree
>>> doc = etree.parse('/path/to/file.xml')
>>> print doc.xpath('Placemark') # Returns an empty list

According to the xpath syntax, the above should select all nodes with the tag 'Placemark'.

>>> print doc.xpath('//Placemark') # Returns an empty list

According to the same source, that should select all nodes no matter where they are in the document.

>>> print doc.xpath('/kml') # Returns an empty list

Again, this should select the root node... Nothing works!

Well, this works:

>>> print doc.xpath('/*') # Returns the kml node
>>> print doc.xpath('/*/*') # Returns the Document node

OK, so I know that final one is not how you are supposed to get the Document node, but since we have it with that, I try to start there and drill down to get the coordinates:

>>> print doc.xpath('/*/*/Folder/Folder/Folder/Document/Folder/Placemark/MultiGeometry/coordinates') # Returns an empty list

I've tried a lot of other things too. Why does nothing but the slash-star syntax seem to work?? Yes, I've tried lots of StackOverflow searches, and people give toy xml files and some code, and I paste it in and it works. But why can't I get any good results with my file?

Ultimately I'm trying to extract these nodes:

/kml/Document/Folder/Folder/Folder/Document/description
/kml/Document/Folder/Folder/Folder/Document/Folder/Placemark/MultiGeometry/coordinates