  We present Chandra ACIS observations of N157B, a young supernova remnant
located in the 30 Doradus star-formation region of the LMC. This remnant
contains the most energetic pulsar known (PSR J0537-6910), which is surrounded
by a bright nonthermal nebula that likely represents a toroidal pulsar wind
terminal shock observed edge-on. We confirm the non-thermal nature of the
comet-shaped X-ray emission feature and show that the spectral steepening of
this feature away from the pulsar is quantitatively consistent with synchrotron
cooling of shocked pulsar wind particles flowing downstream at a bulk velocity
close to the speed of light. Around the cometary nebula we unambiguously detect
a thermal component, which accounts for about 1/3 of the total 0.5 - 10 keV
flux from the remnant. This thermal component is distributed among various
clumps of metal-enriched plasma embedded in the low surface brightness
X-ray-emitting diffuse gas. The relative metal enrichment pattern suggests that
the mass of the supernova progenitor is >~ 20M_sun. A comparison of the X-ray
data with HST optical images suggests that the explosion site is close to a
dense cloud, against which a reflection shock is launched. The interaction
between this reflection shock and the nebula has likely produced both its
cometary shape and the surrounding thermal emission enhancement. SNR N157B is
apparently expanding into the hot low-density interior of a surrounding
superbubble formed by the young OB association LH99, as revealed by Spitzer
mid-infrared images. This scenario naturally explains the exceptionally large
sizes of both the thermal and nonthermal components as well as the lack of an
outer shell of the SNR. These results provide a rare glimpse into the SNR
structure and evolution in a region of recent star-formation.
